ctdb: add optional behaviour to disable ctdb before shutting down

Users can set the option CTDB_DISABLE_BEFORE_SHUTDOWN to yes to enable this.

By default this waits CTDB_DISABLE_BEFORE_SHUTDOWN_SLEEPTIME seconds before shutting down the node, the default is 10 seconds.

This is useful to make SMB2 Durable Handle failover in a cluster working when a node is administratively shutdown.