corosync: detect network partition based on left_list_entries

Corosync passes a number of leaving node with the parameter
left_list_entries of confchg callback. It can be used for detecting
network partition.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Merge branch 'corosync-leave' of https://github.com/sheepdog/sheepdog into corosync-leave

Conflicts:
	sheep/cluster/corosync.c