Generate a unique multicast address for each proposal

We could do that for the multicast port too, but reading on the web, it
seems people more common to use multicast addresses for this (and this
helps avoid bugs that would be caused caused by the fact that we need
two ports for corosync, not just one)
Allow multiple proposals
Merge pull request #24 from vuntz/multi-proposal

Allow multiple proposals for pacemaker barclamp