Migrate scenario utils to tempest client

All scenario tests have been migrated to tempest client.
Migrate scenario utils as well, so that we may drop
the OfficialClientManager completely.

Change-Id: I3ec80de961ba289734f9bc811e4789e7d1b5903c
Partially-implements: bp/tempest-client-scenarios
Merge "Migrate scenario utils to tempest client"