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
Merge "Stop keystone resource leaking even if an error occurs"