Update kerberos client variable names
During review of the kerberos-client role these variable names
changed, but I forgot to update the change that added this
(I9a3af36048e4a4439fe86b36582667ff0af68035).  Update to the prefixed
names.
hosts: all
roles:
- role: kerberos-client
- realm: 'OPENSTACK.ORG'
- admin_server: 'kdc.openstack.org'
- kdcs:
+ kerberos_realm: 'OPENSTACK.ORG'
+ kerberos_admin_server: 'kdc.openstack.org'
+ kerberos_kdcs:
- kdc01.openstack.org
- kdc02.openstack.org
