CHANGELOG: add missing default setting for grpc gateway flag
CHANGELOG: add missing default setting for grpc gateway flag

Add missing changelog entry for the flag setting for the
enable-grpc-gateway.

mkumatag/etcd,etcd-io/etcd,mqliang/etcd,rnaveiras/etcd,etcd-io/etcd,mkumatag/etcd,gyuho/etcd,mkumatag/etcd,mqliang/etcd,ajityagaty/etcd,rnaveiras/etcd,coreos/etcd,coreos/etcd,gyuho/etcd,ajityagaty/etcd,etcd-io/etcd,rnaveiras/etcd,mqliang/etcd,ajityagaty/etcd