Update TEMPEST_README.txt
Update TEMPEST_README.txt

Change I4bf4a02890a33c4034e4493d1763ed4019fdf46e introduced
using a venv to tempest for running the tests. However,
etc/TEMPEST_README.txt still listed it as a TODO. This
updates the readme with the new behavior.

Change-Id: I730b53996f2785b09f02d014f71e6b97d1e20e75

NexusIS/tempest,itskewpie/tempest,yamt/tempest,Juraci/tempest,yamt/tempest,rzarzynski/tempest,armando-migliaccio/tempest,jamielennox/tempest,ebagdasa/tempest,jamielennox/tempest,itskewpie/tempest,tudorvio/tempest,citrix-openstack-build/tempest,vmahuli/tempest,jaspreetw/tempest,Mirantis/tempest,LIS/lis-tempest,adkerr/tempest,manasi24/jiocloud-tempest-qatempest,varunarya10/tempest,tonyli71/tempest,hayderimran7/tempest,bigswitch/tempest,openstack/tempest,rakeshmi/tempest,CiscoSystems/tempest,afaheem88/tempest_neutron,cisco-openstack/tempest,zsoltdudas/lis-tempest,redhat-cip/tempest,citrix-openstack/build-tempest,akash1808/tempest,roopali8/tempest,manasi24/jiocloud-tempest-qatempest,varunarya10/tempest,xbezdick/tempest,vedujoshi/tempest,vmahuli/tempest,bigswitch/tempest,ntymtsiv/tempest,sebrandon1/tempest,tudorvio/tempest,masayukig/tempest,Vaidyanath/tempest,ebagdasa/tempest,hayderimran7/tempest,alinbalutoiu/tempest,xbezdick/tempest,hpcloud-mon/tempest,armando-migliaccio/tempest,hpcloud-mon/tempest,queria/my-tempest,ntymtsiv/tempest,rakeshmi/tempest,sebrandon1/tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,CiscoSystems/tempest,cisco-openstack/tempest,neerja28/Tempest,pczerkas/tempest,alinbalutoiu/tempest,rzarzynski/tempest,izadorozhna/tempest,BeenzSyed/tempest,flyingfish007/tempest,eggmaster/tempest,zsoltdudas/lis-tempest,redhat-cip/tempest,Mirantis/tempest,neerja28/Tempest,adkerr/tempest,akash1808/tempest,cloudbase/lis-tempest,citrix-openstack/build-tempest,BeenzSyed/tempest,nunogt/tempest,pandeyop/tempest,jaspreetw/tempest,Lilywei123/tempest,Juniper/tempest,danielmellado/tempest,Tesora/tesora-tempest,tonyli71/tempest,roopali8/tempest,Vaidyanath/tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,LIS/lis-tempest,citrix-openstack-build/tempest,pandeyop/tempest,Juniper/tempest,JioCloud/tempest,Lilywei123/tempest,manasi24/tempest,dkalashnik/tempest,Tesora/tesora-tempest,vedujoshi/tempest,vedujoshi/os_tempest,afaheem88/tempest,masayukig/tempest,izadorozhna/tempest,NexusIS/tempest,afaheem88/tempest,afaheem88/tempest_neutron,manasi24/tempest,danielmellado/tempest,openstack/tempest,eggmaster/tempest,flyingfish007/tempest,dkalashnik/tempest,Juraci/tempest,nunogt/tempest,JioCloud/tempest,cloudbase/lis-tempest,pczerkas/tempest,vedujoshi/os_tempest,queria/my-tempest