e2e: add watch tests with ETCDCTL_WATCH_*
e2e: add watch tests with ETCDCTL_WATCH_*

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

sinsharat/etcd,mqliang/etcd,etcd-io/etcd,heyitsanthony/etcd,sinsharat/etcd,fanminshi/etcd,rnaveiras/etcd,gyuho/etcd,joshix/etcd,joshix/etcd,bdarnell/etcd,lorneli/etcd,cockroachdb/etcd,lorneli/etcd,coreos/etcd,mqliang/etcd,xiang90/etcd,xiang90/etcd,rnaveiras/etcd,heyitsanthony/etcd,vimalk78/etcd,gyuho/etcd,whuwxl/etcd,li-ang/etcd,coreos/etcd,ajityagaty/etcd,ajityagaty/etcd,etcd-io/etcd,cockroachdb/etcd,dannysauer/etcd,dannysauer/etcd,vimalk78/etcd,li-ang/etcd,glevand/coreos--etcd,bdarnell/etcd,etcd-io/etcd,fanminshi/etcd,mkumatag/etcd,sebrandon1/etcd,whuwxl/etcd,glevand/coreos--etcd,mkumatag/etcd,ajityagaty/etcd,mqliang/etcd,mkumatag/etcd,sebrandon1/etcd,rnaveiras/etcd