Adding missing snapshot_metadata RBAC tests

Adding misssing tests for following:
1)update_snapshot_metadata
2)update_snapshot_metadata with a given key
3)show_snapshot_metadata with a given key
4)delete_snapshot_metadata with a given key

Adding misssing update_snapshot_metadata RBAC test for volume tests
in policy file[1].

[1]https://github.com/openstack/patrole/blob/master/patrole_tempest_plugin/tests/api/volume/test_snapshots_metadata_rbac.py

Change-Id: I71f67a1c206fcd541bb75c602854a28e9280b63a
1 file changed