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.
Merge pull request #105 from dirkmueller/master

Fix keystone admin url (bnc#907103)