Add coverage for trusts parser.Stack delete path

Related-Bug: #1247200
(cherry picked from commit 9904be6febc4acd39fb86afe119aa6427e890b9a)
Change-Id: Ic55030be389ac71ec999e08533fa9d5fc05b5bd1
Catch error deleting trust on stack delete

When deleting a stack, it's possible for deleting the trust to fail,
for example if the user deleting the stack is not the user who created
it, or an admin (which raises a Forbidden error), or due to some
other transient error e.g connection to keystone interrupted.

Currently in this case, we fail to mark the stack deleted in the DB
and leave the status "DELETE, COMPLETE", which is misleading.

Conflicts:
	heat/tests/test_parser.py

Closes-Bug: #1247200
(cherry picked from commit 214ba503757e5bd9bf5a5fab6692c4e94d0536fa)
Change-Id: Ie8e9ea48bc4f44e56ff4764123fcca733f5bd458