run dstat with run_process
run dstat with run_process

It is not clear to me why this can't use run_process?  Currently we
end up with two log-files both with the same thing

- dstat.txt.gz which comes from the "tee" and is symlinked into
  SCREEN_LOGDIR, so gets picked-up by the gate scripts
- screen-dstat.txt.gz which comes from screen_it

Change-Id: I00b9e09b8d44f72ff14e69dc6e4a4bd5e2a0439e

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