Fix volume_glance_metadata deletion (backport)

Backport patch https://review.openstack.org/#/c/41768/ and
its dependency: https://review.openstack.org/#/c/41601/
(no snapshot handling exception, for this is not trivial
due to specific havana changes)

If volume_glance_metadata does not exist for a certain volume,
it returns an exception and does not delete volume from db.
Avoid deleting it if it does not exist.

Fixes bug: 1209367

Change-Id: Icb34c90b13aac1e92b4645c3016127a2d4b8af6f
Merge "Fix volume_glance_metadata deletion (backport)" into stable/grizzly