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
Merge "Call clear_isolated_creds in scenario tests"