e2e: poll '/version' in release upgrade tests
e2e: poll '/version' in release upgrade tests

Fix https://github.com/coreos/etcd/issues/7065.

qqsun8819/etcd,plasticbox/etcd,mqliang/etcd,ajityagaty/etcd,lumjjb/etcd,rnaveiras/etcd,xiang90/etcd,visheshnp/etcd,insequent/etcd,handsomesun/etcd,rchicoli/etcd,tguangch/etcd,CrazyJvm/etcd,fanminshi/etcd,etcd-io/etcd,sebrandon1/etcd,ravigadde/etcd,mqliang/etcd,xiang90/etcd,jlamillan/etcd,sinsharat/etcd,lorneli/etcd,glevand/coreos--etcd,mkumatag/etcd,bluebreezecf/etcd,mickep76/etcd,dannysauer/etcd,joshix/etcd,gyuho/etcd,a-robinson/etcd,bluebreezecf/etcd,cockroachdb/etcd,vimalk78/etcd,yudai/etcd,rnaveiras/etcd,akauppi/etcd,li-ang/etcd,gyuho/etcd,rnaveiras/etcd,coreos/etcd,YuleiXiao/etcd,whuwxl/etcd,plasticbox/etcd,armstrongli/etcd,bdarnell/etcd,sinsharat/etcd,mkumatag/etcd,lorneli/etcd,YuleiXiao/etcd,li-ang/etcd,mkumatag/etcd,vimalk78/etcd,mqliang/etcd,heyitsanthony/etcd,insequent/etcd,etcd-io/etcd,etcd-io/etcd,handsomesun/etcd,ajityagaty/etcd,coreos/etcd,mickep76/etcd,qqsun8819/etcd,sebrandon1/etcd,joshix/etcd,CrazyJvm/etcd,akauppi/etcd,fanminshi/etcd,jlamillan/etcd,dannysauer/etcd,bdarnell/etcd,ajityagaty/etcd,heyitsanthony/etcd,visheshnp/etcd,armstrongli/etcd,whuwxl/etcd,tguangch/etcd,cockroachdb/etcd,rchicoli/etcd,lumjjb/etcd,yudai/etcd,a-robinson/etcd,glevand/coreos--etcd,ravigadde/etcd