onnode: update algorithm for finding nodes file.

onnode: update algorithm for finding nodes file.

2 changes:

* If a relative nodes file is specified via -f or $CTDB_NODES_FILE but
  this file does not exist then try looking for the file in /etc/ctdb
  (or $CTDB_BASE if set).

* If a nodes file is specified via -f or $CTDB_NODES_FILE but this
  file does not exist (even when checked as per above) then do not
  fall back to /etc/ctdb/nodes ((or $CTDB_BASE if set).  The old
  behaviour was surprising and hid errors.

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