Fix
=== RUN TestEmbedEtcd
==================
WARNING: DATA RACE
Write at 0x000001df86d0 by goroutine 711:
github.com/coreos/etcd/embed.(*Config).setupLogging.func1()
/go/src/github.com/coreos/etcd/vendor/google.golang.org/grpc/grpclog/loggerv2.go:68 +0x16c
sync.(*Once).Do()
/usr/local/go/src/sync/once.go:44 +0xe1
github.com/coreos/etcd/embed.(*Config).setupLogging()
in gRPC proxy tests.
Signed-off-by: Gyuho Lee <gyuhox@gmail.com>