Don't delete trust on backup stack delete

Since the trust_id is copied from the stack being backed-up, we
don't want to delete the trust when the backup is deleted, or
the actual stack will break when the next deferred auth operation
occurs.

Closes-Bug: #1271968
(cherry picked from commit 8ce9a177c47b3ee224200690d7181c784c418292)

Conflicts:

	heat/engine/parser.py

Change-Id: I578d4e373d425cc0452ae235637cc50bac455433