Handle missing temp volume and snapshot during backup cleanup

[Xing Yang]

When backup service is started, we try to clean up temp volumes and snapshots in previously failed backups. If the temp volume or snapshot is already deleted, we'll get VolumeNotFound or SnapshotNotFound error. These should be handled. Also temp_volume_id and temp_snapshot_id should be set to None after they are deleted.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/212962

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/212962
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=454bdf0f0fc5d0eb58fa87855700995345c93fc6
Submitter: Jenkins
Branch: master

commit 454bdf0f0fc5d0eb58fa87855700995345c93fc6
Author: Xing Yang <email address hidden>
Date: Sun Aug 9 22:41:21 2015 -0400

    Handle missing temp volume and snapshot during cleanup

    When backup service is started, we try to clean up temp volumes
    and snapshots in previously failed backups. If the temp volume
    or snapshot is already deleted, we will get VolumeNotFound or
    SnapshotNotFound exceptions. These exceptions should be handled.
    Also temp_volume_id and temp_snapshot_id should be set to None
    after they are deleted.

    Change-Id: Ia20834bcc89040364bce71fc66c32c1777a5ac11
    Closes-Bug: #1484774