doc: add TLS examples to clustering guide
doc: add TLS examples to clustering guide

Fixes #3595

joshix/etcd,bdarnell/etcd,li-ang/etcd,glycerine/etcd,qqsun8819/etcd,xiang90/etcd,tguangch/etcd,fanminshi/etcd,bluebreezecf/etcd,gyuho/etcd,etcd-io/etcd,mqliang/etcd,lumjjb/etcd,mkumatag/etcd,a-robinson/etcd,purpleidea/etcd,ravigadde/etcd,jlamillan/etcd,cockroachdb/etcd,YuleiXiao/etcd,dannysauer/etcd,luxas/etcd,lumjjb/etcd,chipironcin/etcd,lorneli/etcd,ktateish/etcd,rnaveiras/etcd,wantonsolutions/dinv-etcd,plasticbox/etcd,swingbach/etcd,tguangch/etcd,rnaveiras/etcd,OyTao/etcd_learning,bdarnell/etcd,armstrongli/etcd,akauppi/etcd,YuleiXiao/etcd,zaphod-concur/etcd,vimalk78/etcd,mqliang/etcd,sebrandon1/etcd,jonboulle/etcd,mkumatag/etcd,sofuture/etcd,CrazyJvm/etcd,qqsun8819/etcd,glevand/coreos--etcd,ajityagaty/etcd,endocode/etcd,glycerine/etcd,xiang90/etcd,gyuho/etcd,chipironcin/etcd,linuxcer/etcd,lorneli/etcd,whuwxl/etcd,mickep76/etcd,ajityagaty/etcd,yudai/etcd,wantonsolutions/dinv-etcd,ravigadde/etcd,coreos/etcd,linuxcer/etcd,insequent/etcd,dannysauer/etcd,etcd-io/etcd,wantonsolutions/dinv-etcd,mickep76/etcd,sofuture/etcd,handsomesun/etcd,li-ang/etcd,nekto0n/etcd,yudai/etcd,purpleidea/etcd,nekto0n/etcd,handsomesun/etcd,ktateish/etcd,rchicoli/etcd,dnaeon/etcd,zaphod-concur/etcd,visheshnp/etcd,vimalk78/etcd,coreos/etcd,StorReduce/etcd,ajityagaty/etcd,rnaveiras/etcd,hongchaodeng/etcd,westhood/etcd,endocode/etcd,luxas/etcd,glevand/coreos--etcd,bluebreezecf/etcd,mqliang/etcd,westhood/etcd,jlamillan/etcd,rchicoli/etcd,swingbach/etcd,etcd-io/etcd,StorReduce/etcd,insequent/etcd,jonboulle/etcd,joshix/etcd,a-robinson/etcd,hongchaodeng/etcd,whuwxl/etcd,OyTao/etcd_learning,fanminshi/etcd,akauppi/etcd,mkumatag/etcd,plasticbox/etcd,cockroachdb/etcd,armstrongli/etcd,CrazyJvm/etcd,sebrandon1/etcd,sinsharat/etcd,dnaeon/etcd,heyitsanthony/etcd,sinsharat/etcd,heyitsanthony/etcd,visheshnp/etcd