Add Keystone role and service test cases

Add two test cases about Keystone role and service:
test_get_role_by_id
test_create_service_without_description

Change-Id: I5c09e82bb9afbb6d1d8a81dfd6e7408b4ffdd58f
diff --git a/tempest/services/identity/json/identity_client.py b/tempest/services/identity/json/identity_client.py
index 99b4036..4a6cdad 100644
--- a/tempest/services/identity/json/identity_client.py
+++ b/tempest/services/identity/json/identity_client.py
@@ -49,6 +49,12 @@
         resp, body = self.post('OS-KSADM/roles', post_body)
         return resp, self._parse_resp(body)
 
+    def get_role(self, role_id):
+        """Get a role by its id."""
+        resp, body = self.get('OS-KSADM/roles/%s' % role_id)
+        body = json.loads(body)
+        return resp, body['role']
+
     def create_tenant(self, name, **kwargs):
         """
         Create a tenant