KeyError: 'backup_id' raised when stack deletion

[huangtianhua]

1. create a stack with cinder volume
2. snapshot for stack, because there is no cinder-backup service, so stack went to 'SNAPSHOT_FAILED'
3. stack-delete, then a keyerror raised, details: http://paste.openstack.org/show/293239/
4. find that bug/1448842, the change Id7441f0c9d537b417488c2857019802872239b2e, has fixed this problem already, but then the
   the change I13f8e03ff090dd5f5739e5af231c77d066eb9eac modify the code, and this problem come again.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/191702

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/191702
Committed: https://git.openstack.org/cgit/openstack/heat/commit/?id=f50013287966e7e6fbe6d0340d97036a3abe35f5
Submitter: Jenkins
Branch: master

commit f50013287966e7e6fbe6d0340d97036a3abe35f5
Author: huangtianhua <email address hidden>
Date: Mon Jun 15 16:38:25 2015 +0800

    Fix KeyError exception when stack snapshot deletion

    There is no 'backup_id' data in snapshot if the cinder
    volume snapshot failed. This patch will use 'get'
    instead __getitem__ to get the 'backup_id' data to avoid
    KeyError exception raise.

    Change-Id: Iae7e22ccdaf8f48a85d90afac3a38011f14cbd34
    Closes-Bug: #1465173