ipaclient: add client-side command override class
    
    This adds a new ipaclient.frontend module with two classes, CommandOverride
    and MethodOverride, which can be used to implement additional client-side
    functionality on top of server-side Command and Method plugins.
    
    https://fedorahosted.org/freeipa/ticket/4739
    
    Reviewed-By: David Kupka <dkupka@redhat.com>