Merge VTTOP and VTROOT variables, setting VTROOT to what was VTTOP

[morgo]

Fixes #5378

The motivation for this patch is that it allows one to checkout the source code, and run make build and then make test with no configuration required.

VTROOT is unfortunately still required for runtime, since configuration files and webfiles need to be located. We can fix this in #5502

The Dockerfiles use the historical layout still, with a symlink from the new 'bin' location back to /vt/bin.

The Docker test runner has been modified so that it will work without waiting for new docker images to be published. After this PR merges and we push new images, there is code in docker/test/run.sh which can be removed.

Note: This also disables the prepared statement test. See: #5524

Backwards Compatibility

This PR is intended to be mostly backwards compatible, however users will need to update their environment variables to point $VTROOT to the correct location (what was VTTOP).

This is not a problem in build or test, since build.env always overwrites $VTROOT and sets it to $PWD. But it is a problem in the examples/ since there are scripts which will attempt to include $VTROOT/web/vtctld. My hope is that we can fix this in #5502 before release, so that $VTROOT is not a run-time requirement at all.

Documentation

The changes to the build guides will be documented in vitessio/website#372

[morgo]

Pasting the result of make docker_bootstrap && go run test.go -pull=false -use_docker_cache=true :

2019/12/06 20:59:58 ============================================================
2019/12/06 20:59:58 mysql57.backup                          	PASS
2019/12/06 20:59:58 mysql57.backup_mysqlctld                	PASS
2019/12/06 20:59:58 mysql57.backup_only                     	PASS
2019/12/06 20:59:58 mysql57.backup_transform                	PASS
2019/12/06 20:59:58 mysql57.backup_transform_mysqlctld      	PASS
2019/12/06 20:59:58 mysql57.binlog                          	PASS
2019/12/06 20:59:58 mysql57.cache_invalidation              	PASS
2019/12/06 20:59:58 mysql57.cell_aliases                    	PASS
2019/12/06 20:59:58 mysql57.cell_no_aliases                 	PASS
2019/12/06 20:59:58 mysql57.check_make_parser               	PASS
2019/12/06 20:59:58 mysql57.client_test                     	PASS
2019/12/06 20:59:58 mysql57.cluster_endtoend                	PASS
2019/12/06 20:59:58 mysql57.e2e_race                        	PASS
2019/12/06 20:59:58 mysql57.encrypted_replication           	PASS
2019/12/06 20:59:58 mysql57.encrypted_transport             	PASS
2019/12/06 20:59:58 mysql57.endtoend                        	PASS
2019/12/06 20:59:58 mysql57.initial_sharding                	PASS
2019/12/06 20:59:58 mysql57.initial_sharding_bytes          	PASS
2019/12/06 20:59:58 mysql57.initial_sharding_multi          	FLAKY (1/2 failed)
2019/12/06 20:59:58 mysql57.java                            	PASS
2019/12/06 20:59:58 mysql57.keyrange                        	PASS
2019/12/06 20:59:58 mysql57.keyspace                        	PASS
2019/12/06 20:59:58 mysql57.legacy_resharding               	PASS
2019/12/06 20:59:58 mysql57.local_example                   	PASS
2019/12/06 20:59:58 mysql57.merge_sharding                  	PASS
2019/12/06 20:59:58 mysql57.merge_sharding_bytes            	PASS
2019/12/06 20:59:58 mysql57.messaging                       	PASS
2019/12/06 20:59:58 mysql57.mysql_server                    	PASS
2019/12/06 20:59:58 mysql57.mysqlctl                        	PASS
2019/12/06 20:59:58 mysql57.prepared_statement              	FAIL (3 tries)
2019/12/06 20:59:58 mysql57.python_client                   	PASS
2019/12/06 20:59:58 mysql57.recovery                        	PASS
2019/12/06 20:59:58 mysql57.reparent                        	PASS
2019/12/06 20:59:58 mysql57.resharding                      	PASS
2019/12/06 20:59:58 mysql57.resharding_bytes                	PASS
2019/12/06 20:59:58 mysql57.resharding_rbr                  	PASS
2019/12/06 20:59:58 mysql57.schema                          	PASS
2019/12/06 20:59:58 mysql57.sharded                         	PASS
2019/12/06 20:59:58 mysql57.sharded_recovery                	PASS
2019/12/06 20:59:58 mysql57.tabletmanager                   	PASS
2019/12/06 20:59:58 mysql57.tabletmanager_consul            	PASS
2019/12/06 20:59:58 mysql57.tabletmanager_etcd2             	PASS
2019/12/06 20:59:58 mysql57.unit                            	PASS
2019/12/06 20:59:58 mysql57.unit_race                       	FLAKY (2/3 failed)
2019/12/06 20:59:58 mysql57.update_stream                   	PASS
2019/12/06 20:59:58 mysql57.update_stream_rbr               	PASS
2019/12/06 20:59:58 mysql57.vertical_split                  	PASS
2019/12/06 20:59:58 mysql57.vertical_split_rbr              	PASS
2019/12/06 20:59:58 mysql57.vschema                         	PASS
2019/12/06 20:59:58 mysql57.vtctld                          	PASS
2019/12/06 20:59:58 mysql57.vtctld_web                      	PASS
2019/12/06 20:59:58 mysql57.vtgate_buffer                   	PASS
2019/12/06 20:59:58 mysql57.vtgate_utils                    	PASS
2019/12/06 20:59:58 mysql57.vtgatev2                        	PASS
2019/12/06 20:59:58 mysql57.vtgatev3                        	PASS
2019/12/06 20:59:58 mysql57.vttest_sample                   	PASS
2019/12/06 20:59:58 mysql57.worker                          	FAIL (3 tries)
2019/12/06 20:59:58 mysql57.xb_recovery                     	PASS
2019/12/06 20:59:58 mysql57.xtrabackup                      	PASS
2019/12/06 20:59:58 mysql57.xtrabackup_xbstream             	PASS
2019/12/06 20:59:58 ============================================================
2019/12/06 20:59:58 56 PASSED, 2 FLAKY, 2 FAILED, 0 SKIPPED
2019/12/06 20:59:58 Total time: 1h42m8s
exit status 1

Worker and prepared_statements failing is expected, they are both disabled tests.

[morgo]

I am going to close this in favor of #5527 so we don't pollute the commit history so much.