Mjulian/ha firewall integration

As Zentyal is installed by default with firewall, some hacks are needed to deploy zentyal-ha easily.
* A network object containing the HA nodes is created and updated when a node creates/joins/leaves a cluster
* A firewall rule allowing any traffic between HA nodes is created when creating the cluster
* When a node receives a request of a joining node, it has to update its configuration and send it to the rest of the cluster nodes, so they could replicate the new configuration.

Unit tests could be improved, but need some hints there.