Refactor duplicate isolated creds code

The code to set the isolated credentials and the client manager
for the compute and volume base test classes was duplicated. The
duplicated code has been moved to tempest.test.BaseTestCase.

Closes-Bug: #1177411
Change-Id: I70aeda46ab9f44a6726b7e1f256ef511a2f84218
Merge "Refactor duplicate isolated creds code"