Previously we just passed the "existing" kwarg directly to the PUT,
which was then ignored by the API, so PATCH update preview commands
wouldn't work. Now heat supports PATCH preview_update_stack, we
can revise this to use the PATCH method when appropriate.
Requires this heat patch for --existing --dry-run to work when used
together: https://review.openstack.org/#/c/230054
Change-Id: Ib07c0a83edfbef7a08b4af344850d0a791b7704d
Closes-Bug: #1501207