Fix usage of openstack_ceph_rgw_auth

Patch [0] left 2 variables for authentication one is
openstack_swift_auth and the other (inexistent) openstack_ceph_rgw_auth
for the ceph_rgw start_keystone task.

This patch leaves only openstack_ceph_rgw_auth.

Closes-Bug: #1769463

[0] https://github.com/openstack/kolla-ansible/commit/84ade4e1499d0762c8ffa8adb3a2b9003476e99c



Change-Id: I1cc522d91f8258f4ca23afc10a0a2a2b35c1ff68
Signed-off-by: Jorge Niedbalski <jorge.niedbalski@linaro.org>