embed: Clear default initial cluster
embed: Clear default initial cluster

NewConfig() should sets initial cluster from name but we should clear it
in the event that another discovery option has been specified.

Fixes #7516

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