`openstack.NewDerivedDriver` sets client to `openstack.GenericClient`
therefore `rackspace.Client` is getting ignored.
`rackspace.Client.Authenticate` has different semantics than
`openstack.GenericClient.Authenticate`, thus almost all operations with
`rackspace` driver fail at the moment.
Signed-off-by: Amir Mohammad Saied <amirsaied@gmail.com>