scylla_cluster: mark only first node as seed

[bhalevy]

Starting from version 4.3 / 2021.1,
only the first node should be a seed in Scylla

[fruch]

we should test is with all of gating

[bhalevy]

we should test is with all of gating

https://jenkins.scylladb.com/view/master/job/scylla-master/job/byo/job/dtest-byo/52

[bhalevy]

@kbr-scylla there are some interesting failures with this change.
https://jenkins.scylladb.com/view/master/job/scylla-master/job/byo/job/dtest-byo/52/testReport/

This is interesting: https://jenkins.scylladb.com/view/master/job/scylla-master/job/byo/job/dtest-byo/52/testReport/update_cluster_layout_tests/TestUpdateClusterLayout/FullDtest___full_split001___test_add_new_node_while_adding_data_1_streaming_/
I'm not entirely convicened it didn't hit a pre-existing topology bug with RF=1.
I'm able to reprodice locally, even with consistent-topology-changes.

I can't explain:
https://jenkins.scylladb.com/view/master/job/scylla-master/job/byo/job/dtest-byo/52/testReport/internode_ssl_test/TestInternodeSSL/FullDtest___full_split004___test_putget_with_internode_rack_ssl/
https://jenkins.scylladb.com/view/master/job/scylla-master/job/byo/job/dtest-byo/52/testReport/internode_ssl_test/TestInternodeSSL/FullDtest___full_split006___test_putget_with_internode_ssl_with_dc_compression/
https://jenkins.scylladb.com/view/master/job/scylla-master/job/byo/job/dtest-byo/52/testReport/internode_ssl_test/TestInternodeSSL/FullDtest___full_split006___test_putget_with_internode_rack_ssl_with_dc_compression/