Add two new controls to add/delete public ip address from a node at runtime.

Add two new controls to add/delete public ip address from a node at runtime.

The controls only modify the runtime setting of which public addresses a node
can server and does not modify /etc/ctdb/public_addresses.
To make the change permanent you also need to edit /etc/ctdb/public_addresses
manually.

After ip addresses have been added/deleted you need to invoke a recovery
for the ip addresses to be redistributed.