Add a new event "ipreallocated"

Add a new event "ipreallocated"
    This is called everytime a reallocation is performed.

    While STARTRECOVERY/RECOVERED events are only called when
    we do ipreallocation as part of a full database/cluster recovery,
    this new event can be used to trigger on when we just do a light
    failover due to a node becomming unhealthy.

    I.e. situations where we do a failover but we do not perform a full
    cluster recovery.

    Use this to trigger for natgw so we select a new natgw master node
    when failover happens and not just when cluster rebuilds happen.

(This used to be ctdb commit 7f4c591388adae20e98984001385cba26598ec67)