ctdb-server: use the new "nodes list" configuration option

ctdb-server: use the new "nodes list" configuration option

Use the new "nodes list" configuration option. Executing the given path
if the path is prefixed by a `!`. The use case is to decouple the nodes
file from the shared storage, especially in the case where the shared
storage is provided by a vfs module.

For an example, imagine a script that runs `curl` on a URL for a
highly-available web server where the URL provides the content
of the nodes file.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>