Check only for added/deleted metadata entries

Utilize the same methodology as test_volume_metadata and only check
for presence or absence of metadata entries explicitily touched on
the snapshot.

Update_snapshot_metadata requires testing for exact match (it
replaces, not appends).

Change-Id: I2be45ff13997269b39de85d16ccaea3504e972cc
Closes-Bug: 1379460
1 file changed