e2e: unset ETCDCTL_API env var before running u2e tests
e2e: unset ETCDCTL_API env var before running u2e tests

existing ETCDCTL_API env var causes e2e to fail some of its tests.  ETCDCTL_API should not be set before e2e tests start.
the tests themselves should set ETCDCTL_API properly.

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