onnode: add "any" nodespec to select any node with running CTDB.

onnode: add "any" nodespec to select any node with running CTDB.

In testing and other situations (e.g. eventscripts) it is necessary to
select a node where a ctdb command can be run.  The whole idea here is
to avoid nodes where ctdbd is not running and where most ctdb commands
would fail.  This implements a standard way of doing this involving a
recursive onnode command.

There is still a small window for a race, where the selected node is
suddenly shutdown, but this is unavoidable.

Signed-off-by: Martin Schwenke <martin@meltin.net>