ctdb-config: depend on /etc/ctdb/nodes file

ctdb-config: depend on /etc/ctdb/nodes file

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14017

CTDB should start as a disabled unit (systemd) in most of the
distributions and, when trying to enable it for the first time, user
should get an unconfigured, or similar, error.

Depending on /etc/ctdb/nodes file will give a clear direction to final
user on what is needed in order to get cluster up and running. It should
work like previous ENABLED=NO variables in SySV like initialization
scripts.

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