etcdctl: fix exec watch command
etcdctl: fix exec watch command

The previous flag parsing has a small issue. It uses
`recursive == true` and `after-index == 0` to determine
if user specifies the sub flags. This is incorrect since
user can specify `after-index = 0`. Then the flag parsing
would be confused.

This commit explicitly find the `--` in the remaining args
and determine the key and cmdArgs accordingly.

cchamplin/etcd,luxas/etcd,elimisteve/etcd,buaazp/etcd,Yhgenomics/etcd,insionng/etcd,njharman/etcd,fasaxc/etcd,onlyjob/etcd,tangkun75/etcd,chipironcin/etcd,Timer/etcd,gityf/etcd,yyljlyy/etcd,jlamillan/etcd,oscarrr110/etcd,glevand/coreos--etcd,rchicoli/etcd,endocode/etcd,swingbach/etcd,mqliang/etcd,openedbox/etcd,dalanlan/etcd,joshix/etcd,jchauncey/etcd,armstrongli/etcd,onlyjob/etcd,openedbox/etcd,zaphod-concur/etcd,fgbreel/etcd,asim/etcd,betawaffle/etcd,njucslqq/etcd,gouyang/etcd,tguangch/etcd,fasaxc/etcd,nekto0n/etcd,tamird/etcd,mike-lerch/etcd,offscale/etcd,zaphod-concur/etcd,asim/etcd,coreos/etcd,a-robinson/etcd,snailwalker/etcd,heyitsanthony/etcd,achanda/etcd,xdlinear/etcd,combk8s/etcd,cockroachdb/etcd,honeyqa/etcd,armstrongli/etcd,OyTao/etcd_learning,mkumatag/etcd,sebschrader/etcd,SivagnanamCiena/etcd,pauldallen/etcd,shakefu/etcd,etcd-io/etcd,insequent/etcd,sayden/etcd,coreos/etcd,qqsun8819/etcd,linuxcer/etcd,freeloop1114/etcd,offscale/etcd,CrazyJvm/etcd,dnaeon/etcd,jravetch/etcd,polvi/etcd,li-ang/etcd,steveeJ/etcd,shafreeck/etcd,ravigadde/etcd,mitake/etcd,xiang90/etcd,imzhulei/etcd,StorReduce/etcd,Timer/etcd,buaazp/etcd,heyitsanthony/etcd,CrazyJvm/etcd,hongbinz/etcd,TheHippo/etcd,jonboulle/etcd,sebschrader/etcd,fanminshi/etcd,youxidev/etcd,westhood/etcd,StorReduce/etcd,arthurcolle/etcd,akauppi/etcd,beni55/etcd,etcd-io/etcd,es-chow/etcd,sayden/etcd,akauppi/etcd,msanterre/etcd,glycerine/etcd,sinsharat/etcd,plasticbox/etcd,njharman/etcd,iamqizhao/etcd,svdata/etcd,yudai/etcd,spacejam/etcd,kkaneda/etcd,devfreesg/etcd,qqsun8819/etcd,youxidev/etcd,cchamplin/etcd,wowsoso/etcd,ajityagaty/etcd,es-chow/etcd,chenbaihu/etcd,gouyang/etcd,mkumatag/etcd,wantonsolutions/dinv-etcd,arthurcolle/etcd,gityf/etcd,axingblog/etcd,giantswarm/etcd,linuxcer/etcd,rnaveiras/etcd,junxu/etcd,purpleidea/etcd,nkhare/etcd,visheshnp/etcd,siddontang/etcd,chipironcin/etcd,freeloop1114/etcd,insequent/etcd,sebrandon1/etcd,dannysauer/etcd,devfreesg/etcd,njucslqq/etcd,cockroachdb/etcd,jefby/etcd,imzhulei/etcd,msanterre/etcd,fanminshi/etcd,glycerine/etcd,jefby/etcd,yichengq/etcd,ktateish/etcd,kkaneda/etcd,vimalk78/etcd,gyuho/etcd,handsomesun/etcd,lumjjb/etcd,lzbgt/etcd,improbable-io/etcd,dnaeon/etcd,steveeJ/etcd,sebrandon1/etcd,TheHippo/etcd,kingland/etcd,mqliang/etcd,bluebreezecf/etcd,princeqjzh/etcd,spacejam/etcd,dalanlan/etcd,shakefu/etcd,a-robinson/etcd,oscarrr110/etcd,fgbreel/etcd,chenbaihu/etcd,cognusion/etcd,hroyrh/etcd,bdarnell/etcd,Eagle-X/etcd,westhood/etcd,gyuho/etcd,lzbgt/etcd,kingland/etcd,jonboulle/etcd,polvi/etcd,beni55/etcd,sofuture/etcd,hongchaodeng/etcd,ajityagaty/etcd,betawaffle/etcd,honeyqa/etcd,xychu/etcd,hongchaodeng/etcd,cognusion/etcd,delkyd/etcd,raoofm/etcd,jeremyeder/etcd,nkhare/etcd,wowsoso/etcd,ravigadde/etcd,sinsharat/etcd,hongbinz/etcd,combk8s/etcd,Eagle-X/etcd,Yhgenomics/etcd,shawnps/etcd,improbable-io/etcd,handsomesun/etcd,whuwxl/etcd,wantonsolutions/dinv-etcd,tempbottle/etcd,jlamillan/etcd,sofuture/etcd,yichengq/etcd,elimisteve/etcd,princeqjzh/etcd,axingblog/etcd,ktateish/etcd,jravetch/etcd,jeremyeder/etcd,tangkun75/etcd,hroyrh/etcd,giantswarm/etcd,SivagnanamCiena/etcd,wfxiang08/etcd,jchauncey/etcd,shafreeck/etcd,Winslett/etcd,mqliang/etcd,lorneli/etcd,fasaxc/etcd,bluebreezecf/etcd,rnaveiras/etcd,luxas/etcd,plasticbox/etcd,wfxiang08/etcd,YuleiXiao/etcd,purpleidea/etcd,HideoYamauchi/etcd,mickep76/etcd,insionng/etcd,rnaveiras/etcd,glevand/coreos--etcd,xiang90/etcd,gae123/etcd,xychu/etcd,achanda/etcd,visheshnp/etcd,pauldallen/etcd,yudai/etcd,delkyd/etcd,rchicoli/etcd,mitake/etcd,junxu/etcd,yyljlyy/etcd,lumjjb/etcd,endocode/etcd,vimalk78/etcd,xdlinear/etcd,svdata/etcd,li-ang/etcd,joshix/etcd,lorneli/etcd,mickep76/etcd,mkumatag/etcd,snailwalker/etcd,tamird/etcd,dannysauer/etcd,wantonsolutions/dinv-etcd,HideoYamauchi/etcd,swingbach/etcd,nekto0n/etcd,OyTao/etcd_learning,tguangch/etcd,siddontang/etcd,mike-lerch/etcd,gae123/etcd,shawnps/etcd,tempbottle/etcd,Winslett/etcd,YuleiXiao/etcd,iamqizhao/etcd,bdarnell/etcd,ajityagaty/etcd,raoofm/etcd,etcd-io/etcd,whuwxl/etcd