sheep: let gateway node exit in a case of gateway only cluster

When a cluster has gateway nodes only, it means the gateway nodes
doesn't contribute to I/O of VMs. So this patch simply let them exit
and avoid the below recovery issue.

Related issue:
https://bugs.launchpad.net/sheepdog-project/+bug/1327037

Cc: duron800@qq.com
Tested-by: Yang Zhang <3100100878@zju.edu.cn>
Tested-by: Xiaolong Xu <nxtxiaolong@gmail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>