Call clear_isolated_creds in scenario tests

After commit b7144eb08175d010e1300e14f4f75d04d9c63c98, tempest.scenario.manager
doesn't clear isolated credentials anymore. This patch adds a tearDownClass
method to class OfficialClientTest so isolated credentials can be cleared.

Change-Id: Ic03e0d5026e09bff6d1d20daf2ee72bfdb106145
Closes-Bug: #1343027
orchestration api test_stacks wait for deletion

Wait for delete to complete after triggering it, so we avoid
a conflict exception when the test _clear_stacks cleanup tries
to delete it again.

Change-Id: I64065ad8444d814e2cd7f162b869a2f8eedb1fad
Closes-Bug: #1344989
Merge "orchestration api test_stacks wait for deletion"
Merge "Call clear_isolated_creds in scenario tests"