Unicast option for hacluster charm

Very often multicast is not allowed in production environments. Purpose of multicast in corosync is to enable easy discovery of the nodes that want to join the cluster. That seems a bit redundant within juju environment. All the nodes that will form the cluster are already known and given by juju. So, instead of trashing multicast on the network, ideally charm would setup corosync in unicast mode, and node information would be pulled from juju.

What hacluster charm needs to do is replace interface {} section in corosync.conf. When unicast is enabled, that part shoud be like this:

        interface {
                member {
                        memberaddr: $node_1_ip
                }
                member {
                        memberaddr: $node_2_ip
                }
                member {
                        memberaddr: $node_3_ip
                }
                ringnumber: 0
                        bindnetaddr: $bindnetaddr
                        mcastport: $mcastport
                }
                transport: udpu
                }

Replace $variables with proper values. As a side effect this might allow HA within dislocated networks, like AWS.