test: add check for adding nodes to the gateway only cluster

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep: save backend store name even if the node is gateway

A gateway node also has to save the store name to the config file to
notify the name to all the nodes when it becomes master.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>