Stop neutron 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
  +---- image (done)
  +---- network         <-- this patch
  +---- object_storage
  +---- orchestration
  +---- queuing (unnecessary)
  +---- telemetry (unnecessary)
  +---- volume (done)

Partially Implements: blueprint stop-leaking

Change-Id: I580be8f974e3daa8ae4895ebb3e4311be8f9c242
Merge "Stop neutron resource leaking even if an error occurs"