etcdserver: delete member directory when discovery fails

When discovery JoinCluster fails returning DiscoveryError, etcd node falls
back to proxy. And in order to start as a proxy, we need to make sure there
are conflicting directories: 'member' and 'proxy' directories cannot be
existent together. This deletes 'member' directory only when startEtcd fails
with DiscoveryError type. This fixes
etcd-io#3827.

etcdserver/server.go

@@ -272,6 +272,10 @@ func NewServer(cfg *ServerConfig) (*EtcdServer, error) {
 			var err error
 			str, err = discovery.JoinCluster(cfg.DiscoveryURL, cfg.DiscoveryProxy, m.ID, cfg.InitialPeerURLsMap.String())
 			if err != nil {
+				// It removes member directory when NewServer returns error.
+				// This prevents conflicts with 'proxy' directory when
+				// the node falls back to proxy.
+				os.RemoveAll(cfg.MemberDir())
 				return nil, &DiscoveryError{Op: "join", Err: err}
 			}
 			urlsmap, err := types.NewURLsMap(str)