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
Merge "Add tests_ci/pre_test_hook.sh"