Rename tools/pip-requires to requirements.txt and tools/test-requires
to test-requirements.txt. These are standard files, and tools in the
general world are growing intelligence about them.
Change-Id: I903213fda94a833335abaa7ad9a90bbb688ec15a
Fixes: bug #1179008
This syncs install_venv_common.py from oslo and reworks the
tools/install_venv.py script to use the new library.
Change-Id: I3426a7f51b0018e074cc6f4b1d70b38e52464a38