CHANGELOG: add v3.4.15
CHANGELOG: add v3.4.15

Signed-off-by: Gyuho Lee <0fbd30d364d71722b926950b451cecd97b54fc29@amazon.com>

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