Feature make heartbeat configurable

The default heartbeats of 3 seconds are really really short, even too short for VMWare snapshots. 
(See http://linux.die.net/man/5/corosync.conf, as totem is the timeout and not totem * token_retransmits_before_loss_const) 

This change is 100% backward compatible and allows to parametrize the timeouts on the corosync layer.