Take a vm out of SNAPSHOTTING after Glance error

If a snapshot is attempted to be taken when Glance can't find the
underlying image (usually because it's been deleted), then ImageNotFound
is raised to nova.compute.manager.ComputeManager._snapshot_instance. The
error path for this leaves the instance in a snapshotting state; after
which "operator intervention" is required.

This simply reverts the instance state under that specific circumstance.

DocImpact: previously a snapshot that failed for this reason would leave
the instance in a spurious snapshotting task state indefinitely. Now,
the task state is reverted. At the moment that revert is 'silent'
(although the failure to snapshot will be evident to the user).

Change-Id: I9623cfaed31e8337ab65bc1eab85ad3b51b1a5ca
Fixes-bug: #1267097
Merge "Take a vm out of SNAPSHOTTING after Glance error"