[WIP] Add a submodule for /pkg/apis

[carbonin]

This will make the apis easier to consume from outside hive be limiting
the dependencies needed to access them.

Fixes #1279

[openshift-ci-robot]

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[openshift-ci]

@carbonin: The following tests failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
ci/prow/verify	57e8c3a	link	/test verify
ci/prow/e2e	57e8c3a	link	/test e2e

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[carbonin]

Currently stuck on this:

17:06:10:~/Source/hive/pkg/apis (submodule *)$ git grep openshift/hive | grep -v pkg/apis
hive/v1/validating-webhooks/clusterdeployment_validating_admission_hook.go:27:	"github.com/openshift/hive/pkg/constants"
hive/v1/validating-webhooks/clusterdeployment_validating_admission_hook.go:28:	"github.com/openshift/hive/pkg/manageddns"
hive/v1/validating-webhooks/clusterdeployment_validating_admission_hook_test.go:25:	"github.com/openshift/hive/pkg/constants"
hive/v1/validating-webhooks/feature_gates.go:15:	"github.com/openshift/hive/pkg/constants"

We can't have a dependency back into the main module of hive.
The only solution I could come up with is moving the constants and manageddns packages into the apis module, but don't have time to try that today.

[openshift-ci-robot]

@carbonin: PR needs rebase.

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