ctdb-tests: Clean up nodes and public address file setup

ctdb-tests: Clean up nodes and public address file setup

Untangle a single loop into two separate, clear functions.  Create a
separate, empty file for the node with no public IPs instead of
pointing the configuration at /dev/null.

Leave the indentation in setup_ctdb() in the old style to make this
commit comprehensible.

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