Add missing isolated_cred cleanup to savanna tests

This commit adds a missing call to the tearDownClass in the base savanna
test class. Without this call every time the savanna tests run with
tenant isolation enabled the resources created will never be cleaned up.

Closes-Bug: #1275003

Change-Id: I69bafb5a9149f7c49390fc33f27629a8caeb5f9f
Merge "Add missing isolated_cred cleanup to savanna tests"