Migrate test_encrypted_cinder_volumes to tempest client

Migrate the test parent class of EncryptionScenarioTest from
OfficialClientTest to ScenarioTest. And some helper functions are
migrated to ScenarioTest.

Change-Id: I33bcd8d0d787ce67617e6ef07652fe1ca1b7e111
Partially-implements: bp:tempest-client-scenarios
Merge "Migrate test_encrypted_cinder_volumes to tempest client"