Revoke the tokens of group members when a group role is revoked

Fixes a bug introduced by the previous commit:
https://review.openstack.org/#/c/115338/

If revoke_api is not enabled, tokens of group members are not revoked
when a group role is revoked.

Change-Id: I7542ee775ddd6fd240ecb370a3e243e465accbb0
Closes-Bug: 1369664
Merge "Revoke the tokens of group members when a group role is revoked"