install: add restart_dirsrv for directory server restarts
    
    * Create a utility function to restart a directory server and
        reconnect the api.Backend.ldap2 connection.
    * Use restart_dirsrv instead of knownservices.dirsrv.restart to
        ensure api.Backend.ldap2 is reconnected.
    
    https://fedorahosted.org/freeipa/ticket/6461
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>