Fix unstack.sh to stop stop_dstat when it is disabled.
Fix unstack.sh to stop stop_dstat when it is disabled.

unstack.sh always stop dstat process, however unstack.sh show 'dstat: no process
found' when dstat is disabled.
This patch stop function of stop_dstat, when dstat is disabled.

Change-Id: If9054826bed8a7fedd4f77ef4efef2c0ccd7f16e
Closes-Bug: #1435148

bswartz/devstack,dtroyer/devstack,Mahito/devstack,olivierlemasle/devstack,thomasem/devstack,mssumanth/devstack,parul24/devstack,bq-xiao/devstack,prithivm/branch01,samgoon/devstack,HPCHub/devstack,nawawi/openstack,upadhyay-prashant/devstack,kplus/devstack-moonshot,pombredanne/devstack,richliu/devstack,vishnugonela/devstack,dirkmueller/devstack,andrewrothstein/devstack,mmasaki/devstack,parul24/devstack,pombredanne/devstack,pacinete/devstack,neerja28/Devstack_GlusterFS,rushiagr/devstack,samgoon/devstack,bswartz/devstack,HPCHub/devstack,noironetworks/devstack,TwinkleChawla/devstack,avvocatodemarchis/devstack,vsham20/devstack,CloudServer/devstack,herrsechs/devstack,prithivm/branch01,noironetworks/devstack,neerja28/Devstack_GlusterFS,varunarya10/devstack,j4/devstack,pczerkas/devstack,bljgaurav/openstack-test,flavio-fernandes/devstack,openstack-dev/devstack,group-policy/devstack,NaokiMiyata/devstack,vsham20/devstack,nawawi/openstack,herrsechs/devstack,mmasaki/devstack,sequenceiq/devstack,bigswitch/devstack,dirkmueller/devstack,wenhuizhang/devstack,LoHChina/devstack,TwinkleChawla/devstack,openstack-dev/devstack,liuquansheng47/devstack,vishnugonela/devstack,group-policy/devstack,varunarya10/devstack,olivierlemasle/devstack,avvocatodemarchis/devstack,LoHChina/devstack,CloudServer/devstack,dtroyer/devstack,ozamiatin/devstack,bq-xiao/devstack,bigswitch/devstack,nuxeh/devstack,pacinete/devstack,Mahito/devstack,j4/devstack,mc2014/devstack,flavio-fernandes/devstack,mc2014/devstack,costingalan/devstack,nuxeh/devstack,eharney/devstack,andrewrothstein/devstack,wenhuizhang/devstack,kplus/devstack-moonshot,mssumanth/devstack,NaokiMiyata/devstack,liuquansheng47/devstack,atulpatil301/devstack,sequenceiq/devstack,upadhyay-prashant/devstack,richliu/devstack,ozamiatin/devstack,thomasem/devstack,pczerkas/devstack,costingalan/devstack,eharney/devstack,rushiagr/devstack,atulpatil301/devstack,bljgaurav/openstack-test