Code review comment for lp:~mpontillo/maas/update-host-maps-cleanup-bug-1440102

Updated update_nodegroup_host_maps() signature. (it now just takes a list of nodegroups, but the old update_host_maps() will remain the same and ignore the node's nodegroup if passed in, because it breaks test cases without that behavior; even though I'm not positive the behavior is correct, I don't want to risk it.)