Comment 8 for bug 1501207

Reviewed: https://review.openstack.org/231277
Committed: https://git.openstack.org/cgit/openstack/heat/commit/?id=4b30adeae16a908e2415d379fbd262e7913d88da
Submitter: Jenkins
Branch: stable/liberty

commit 4b30adeae16a908e2415d379fbd262e7913d88da
Author: Steven Hardy <email address hidden>
Date: Thu Oct 1 17:16:01 2015 +0100

    Update preview_update_stack to align with PATCH updates

    Currently attempting to do a preview update call with PATCH fails,
    because we didn't align the behavior of preview update with the
    actual update in the recent additions to fix bug #1224828

    So, refactor to ensure both preview_update & update use the same
    code, and add a PATCH path to the update API.

    Change-Id: I8ce5c0ea4035a7b9563db10ea10433e7f5f99a4f
    Closes-Bug: #1501207
    (cherry picked from commit 604595a39c9345d214f9f02c2ab8d7d1d768fcbe)