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.