Improve test_implied_domain_roles

This adds an assertion to verify that global roles can't imply domain
specific ones.

It is also included a new feature flag since the related bug fix
wasn't backported to Mitaka.

Change-Id: Ic7c5b93e4d679c845f546da85984517f79d1b4b7
Depends-On: I70e3ce79ee6d9b00cc48bb178bd423d0196f6588
Related-Bug: 1590578
2 files changed