Stop swift resource leaking even if an error occurs

In some test, if an error occurrs in setUpClass(), some resources
are not deleted because of tearDownClass() is not called.
This commit fixes it but doesn't touch the code of creating only one
resource without waiting for the resource creation.

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

Partially Implements: blueprint stop-leaking

Change-Id: I69dd5bf9dbfa04e463b63f4f236fc0aa9e70e79a
Merge "Stop swift resource leaking even if an error occurs"