ctdb-tests: Rationalise node stop/start/restart

ctdb-tests: Rationalise node stop/start/restart

Separate functions are not needed for stopping/starting/restarting
individual nodes.  The stop and start functions essentially just use
onnode, though for local daemons this is embedded in local_daemons.sh.
So, just provide one stop and one start function that takes an
optional nodespec, defaulting to all nodes.

Restarting becomes common.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>