cache the pip tarball
cache the pip tarball

there is no need to download the pip tarball over and over again
on rerun. Use the cached value.

Change-Id: Ibcc1807ba2aca062f69839212f05ac9394db9448

wenhuizhang/devstack,tianweizhang/devstack,HPCHub/devstack,bljgaurav/openstack-test,richliu/devstack,prithivm/branch01,NaokiMiyata/devstack,openstack-dev/devstack,pombredanne/devstack,avvocatodemarchis/devstack,herrsechs/devstack,olivierlemasle/devstack,NaokiMiyata/devstack,noironetworks/devstack,nawawi/openstack,affo/devstack,jamielennox/devstack,pacinete/devstack,TwinkleChawla/devstack,bljgaurav/openstack-test,richliu/devstack,sc68cal/devstack,BSJAIN92/OpenStack,parul24/devstack,openstack-dev/devstack,dirkmueller/devstack,flavio-fernandes/devstack,bigswitch/devstack,neerja28/Devstack_GlusterFS,bswartz/devstack,kplus/devstack-moonshot,Mahito/devstack,flavio-fernandes/devstack,pacinete/devstack,srics/devstack,atulpatil301/devstack,group-policy/devstack,j4/devstack,neerja28/Devstack_GlusterFS,atulpatil301/devstack,herrsechs/devstack,j4/devstack,LoHChina/devstack,tianweizhang/devstack,costingalan/devstack,andrewrothstein/devstack,mssumanth/devstack,eharney/devstack,andrewrothstein/devstack,mmasaki/devstack,Mahito/devstack,affo/devstack,promptworks/devstack,mc2014/devstack,bigswitch/devstack,mssumanth/devstack,bswartz/devstack,BSJAIN92/OpenStack,noironetworks/devstack,pombredanne/devstack,TwinkleChawla/devstack,LoHChina/devstack,rushiagr/devstack,bq-xiao/devstack,prithivm/branch01,thomasem/devstack,nawawi/openstack,innodee/openstackk,upadhyay-prashant/devstack,nuxeh/devstack,dtroyer/devstack,pczerkas/devstack,vsham20/devstack,promptworks/devstack,varunarya10/devstack,pczerkas/devstack,liuquansheng47/devstack,HPCHub/devstack,bq-xiao/devstack,samgoon/devstack,olivierlemasle/devstack,eharney/devstack,avvocatodemarchis/devstack,dirkmueller/devstack,ozamiatin/devstack,CloudServer/devstack,nuxeh/devstack,samgoon/devstack,sequenceiq/devstack,wenhuizhang/devstack,ozamiatin/devstack,kplus/devstack-moonshot,rushiagr/devstack,vishnugonela/devstack,mmasaki/devstack,vishnugonela/devstack,williamthegrey/devstack,liuquansheng47/devstack,virtualopensystems/devstack,upadhyay-prashant/devstack,dtroyer/devstack,parul24/devstack,Juniper/devstack,sequenceiq/devstack,JioCloud/devstack,thomasem/devstack,group-policy/devstack,sc68cal/devstack,vsham20/devstack,varunarya10/devstack,CloudServer/devstack,mc2014/devstack,eshnil2000/devstack,costingalan/devstack