Increase coverage for volume metadata tests

Implement complete coverage for the following policies [0]:
  * "volume:get_volume_metadata"
  * "volume_extension:volume_image_metadata"

[0] https://github.com/openstack/cinder/blob/0ac4af182fdd55d4a97ce283070f7b8e6480540f/cinder/policies/volume_metadata.py

Change-Id: Id5d9b53392859dd4e852c6aea670334c6acc6670
1 file changed