ctdb: disable the service by default

The systemd service is disabled by default, as the service will fail to start
without /etc/ctdb/nodes. If the user supplies this, they can re-enable the
service.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>