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
diff --git a/patrole_tempest_plugin/tests/api/volume/test_encryption_types_rbac.py b/patrole_tempest_plugin/tests/api/volume/test_encryption_types_rbac.py
index 62ccc9e..f10e41b 100644
--- a/patrole_tempest_plugin/tests/api/volume/test_encryption_types_rbac.py
+++ b/patrole_tempest_plugin/tests/api/volume/test_encryption_types_rbac.py
@@ -82,3 +82,13 @@
         vol_type_id = self._create_volume_type_encryption()
         with self.rbac_utils.override_role(self):
             self.encryption_types_client.show_encryption_type(vol_type_id)
+
+    @decorators.idempotent_id('d4ed3cf8-52b2-4fa2-910d-e405361f0881')
+    @rbac_rule_validation.action(
+        service="cinder",
+        rule="volume_extension:volume_type_encryption")
+    def test_show_encryption_specs_item(self):
+        vol_type_id = self._create_volume_type_encryption()
+        with self.rbac_utils.override_role(self):
+            self.encryption_types_client.show_encryption_specs_item(
+                vol_type_id, 'provider')