Try using machine credentials for RENEW calls

Using user credentials for RENEW calls will fail when the user
credentials have expired.

To avoid this, try using the machine credentials when making RENEW
calls. If no machine credentials have been set, fall back to using user
credentials as before.

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>