Fix keystone admin url (bnc#907103)

The internal_auth_url is pointing to something like :5000/v2.0.
Ceph RadosGW appends /v2.0 there which makes it fail to validate
the keystone token. Use the admin url instead which avoids
the issue.

(cherry picked from commit dae79da757a0d58066d67cd5da6c4ce2cc8230ab)