Syncs install_venv_common.py from oslo-incubator

Needed to make run_tests.sh identify requirements correctly

We need to check requirements.txt and test-requirements.txt
in parallel to ensure both are satisfied and we don't upgrade
a package beyond requirements.txt's limits when installing
dependencies for test-requirements.txt

Fixes bug: 1227019

Change-Id: I606f8a1d3eed5c270d5b3c154994d5c3a17b98c8
Merge "Syncs install_venv_common.py from oslo-incubator"