Add tests_ci/pre_test_hook.sh
Add tests_ci/pre_test_hook.sh

This script is executed inside pre_test_hook function in desvstack gate.
It installs rally devstack integration and exports necessary environment
variables.

Change-Id: Idf6d09ca33337c3b4a32912048653c13f53e2cf3
Related: blueprint rally-gate-jobs

afaheem88/rally,paboldin/rally,group-policy/rally,paboldin/rally,cernops/rally,group-policy/rally,afaheem88/rally,amit0701/rally,openstack/rally,vponomaryov/rally,openstack/rally,varunarya10/rally,vefimova/rally,ytsarev/rally,shdowofdeath/rally,eonpatapon/rally,aforalee/RRally,pandeyop/rally,cernops/rally,eayunstack/rally,pyKun/rally,gluke77/rally,yeming233/rally,pandeyop/rally,gluke77/rally,vganapath/rally,gluke77/rally,aplanas/rally,amit0701/rally,go-bears/rally,group-policy/rally,varunarya10/rally,redhat-openstack/rally,vganapath/rally,eayunstack/rally,vefimova/rally,gluke77/rally,aforalee/RRally,pyKun/rally,eayunstack/rally,aplanas/rally,ytsarev/rally,redhat-openstack/rally,yeming233/rally,amit0701/rally,vponomaryov/rally,openstack/rally,vganapath/rally,paboldin/rally,vganapath/rally,shdowofdeath/rally,go-bears/rally,openstack/rally,eonpatapon/rally