keystone trusts API test, move delete into tearDown handler

We want to prove the delete works for the various configurations of
trusts created in the tests, so move the delete into tearDown instead
of having a cleanup handler and explicit testing of delete in various
tests, and add a comment explaining why we do it like that.

Change-Id: I32f9aee5657ae54c1ca600f6ebe70bda81cd40e2
1 file changed