Skip tests when service barbican is enabled

We shouldn't run this tests when service Barbican enabled
because of they doesn't support Image Signature Verification.

This scenarios verifying in Barbican Tempest Plugin.

list of skipped tests:
tempest.scenario.test_snapshot_pattern.TestSnapshotPattern.test_snapshot_pattern
tempest.scenario.test_encrypted_cinder_volumes.TestEncryptedCinderVolumes.test_encrypted_cinder_volumes_luks
tempest.scenario.test_encrypted_cinder_volumes.TestEncryptedCinderVolumes.test_encrypted_cinder_volumes_cryptsetup
tempest.scenario.test_minimum_basic.TestMinimumBasicScenario.test_minimum_basic_scenario

Change-Id: I1450d3c0921840829f3a1abf6840a7aa6e5b429f
Related-prod: PROD-22487
3 files changed