This reverts Ie518bffe825ae1c6d139a0613c30bc9748b52cd7 and
updates the scripts to properly work with the infra scripts.
It replicates now the current setup done by horizon - thus
moving the babel files to the top-level and adjusting run_tests.sh.
This copies over tools/install_venv_common.py from oslo-incubator so
that the message extraction works with the parameter that the infra
scripts expect.
The infra scripts have been adopted to work with this setup, see
change I09350bfd384f84d132dc03d9bae2a5015acb6b0f.
Change-Id: I6f5c8a4dc8897b27f9f700be42d5dd38f7d278d0