Add RBAC test for show_encryption_specs_item

This adds an RBAC test for volume v3 client endpoint
show_encryption_specs_item. This was recently added to Tempest
in [0]. The policy it enforces is
"volume_extension:volume_type_encryption" [1] and it is enforced
in code here [2].

[0] https://review.openstack.org/#/c/521241/
[1] https://github.com/openstack/cinder/blob/e75d209177b8b65f28429e64e79b4101dbcf58b3/cinder/policies/volume_type.py#L65
[2] https://github.com/openstack/cinder/blob/e75d209177b8b65f28429e64e79b4101dbcf58b3/cinder/api/contrib/volume_type_encryption.py#L147

Change-Id: I8518bee5720956eb6c3c280e311458991ff30325
1 file changed