dog: add argument for cluster info to display backend store type

'dog cluster info' does not show backend store information, so we
add argument '-s' to display it.

Now, when using 'dog cluster info -s', it will display:

	Cluster status: running, auto-recovery enabled
	Cluster store: plain with 8:15 redundancy policy

	Cluster created at Wed Dec  4 16:39:21 2013

	Epoch Time           Version
	......

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
dog: check nr_copies before format cluster

If we use erasure-code for "4:2" and there are only 4 nodes in cluster, it will
format cluster successly but crash when writing data into vdi.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: add check to avoid accessing array out of range

If we use 4:2 as copy_policy and there are 6 nodes in cluster, everything is ok.
But after one node crash, the target_nodes[] will contain just 5 elements, when
gateway_forward_request() access the target_nodes[] with max index 'nr_to_send'
(which is 6), it will cause problem.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>