Add Show ' update_backup ' policy tests

Adding missing RBAC test for volume backup update
in policy file[1], api-ref[2]

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

[2]https://github.com/openstack/cinder/blob/master/cinder/policies/backups.py#L80

Change-Id: I1189f53f32b5008200dbf2b67e349577973065df
1 file changed