cluster-mgmt: update discovery doc for etcd2
cluster-mgmt: update discovery doc for etcd2

mischief/coreos-docs,michelleN/docs,jdloft/docs,jonboulle/docs,atxwebs/docs,skynet/docs,wli6/docs,endocode/coreos-docs,bq-xiao/docs,joshix/docs,marineam/coreos-docs,robszumski/docs,ionave/docs,coreos/docs,iankronquist/docs,hkjn/docs-1,mjg59/docs