backup init_host cleanup volume detach should be resilient and effective

[Tom Barron]

In review https://review.openstack.org/#/c/210147/ for the fix for https://bugs.launchpad.net/cinder/+bug/1482548 two issues were raised that need to be fixed though they were outside the scope of that fix:

1. current code is not resilient to detach failures of a particular volume. On such failures it should skip DB update, and move onto the next volume.

2. current code simply issues 'detach_volume', which basically just updates the DB, rather than doing everything needed to actually detach the volume.

This bug is for followup action to address these remaining issues.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/221889

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/221889
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=bda6da1f8df44a64e8b320ebd8e2211197b6afeb
Submitter: Jenkins
Branch: master

commit bda6da1f8df44a64e8b320ebd8e2211197b6afeb
Author: Tom Barron <email address hidden>
Date: Wed Aug 26 16:07:00 2015 -0400

    backup init_host cleanup exception handling

    Current cleanup of leftover backups, volumes, and temporary
    volumes and snapshots is not resilient in the face of exceptions
    such as failures in detach operations.

    This commit adds appropriate try/except logic to this section
    of code, thereby ensuring that the backup service can start even
    if exceptions are encountered when attempting to cleanup detritus
    from earlier failed backup operations.

    Change-Id: Ie8d2107ebd4d86dfe5664c35687e8cbe58cfad9d
    Closes-bug: 1485295