cluster: gracefully shutdown gRPC server
cluster: gracefully shutdown gRPC server

Fix https://travis-ci.org/coreos/etcdlabs/builds/226865093?utm_source=email&utm_medium=notification.

Signed-off-by: Gyu-Ho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

gyuho/etcdlabs,gyuho/etcdlabs,gyuho/etcdlabs,gyuho/etcdlabs,gyuho/etcdlabs