Allow services to be members of the groups, like users and other groups
can already be.
This is required for use cases where such services aren't associated
with a particular host (and thus, the host object cannot be used to
retrieve the keytabs) but represent purely client Kerberos principals to
use in a dynamically generated environment such as Kubernetes.
Fixes: https://pagure.io/freeipa/issue/7513
Reviewed-By: Rob Crittenden <rcritten@redhat.com>
e642865717