etcdmain: create self-signed certs when listening on https for httpproxy
etcdmain: create self-signed certs when listening on https for httpproxy

Fixes failures from TestCtlV3PutClientAutoTLS in proxy coverage tests.

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