Stop keystone resource leaking even if an error occurs

In some test, if an error occurs in setUpClass(), some resources
are not deleted because of tearDownClass() is not called.
This commit fixes it.

 tempest/api
  +---- baremetal (unnecessary)
  +---- compute
  +---- data_processing (unnecessary)
  +---- database (unnecessary)
  +---- identity            <-- this patch
  +---- image (done)
  +---- network
  +---- object_storage
  +---- orchestration
  +---- queuing (unnecessary)
  +---- telemetry (unnecessary)
  +---- volume (done)

Partially Implements: blueprint stop-leaking

Change-Id: I7104b968b7c704d465ec8a5a3046f749566c1241
4 files changed