s3:script: clustered samba: Add script updatekeytab.sh

s3:script: clustered samba: Add script updatekeytab.sh

Admin should use this script in smb.conf parameter 'sync machine
password script' in clustered samba

TODO: onnode will update the keytab on all connected nodes, so the
update will happen on the triggering node twice. This can be improved in
the future.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=6750

Signed-off-by: Pavel Filipenský <pfilipensky@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>