Updated README to reflect latest changes.

Change-Id: I996d5c0ec6e480fcbdc546bad238eee2e20504d6
Use install_venv_common.py from oslo.

This syncs install_venv_common.py from oslo and reworks the
tools/install_venv.py script to use the new library.

Change-Id: I3426a7f51b0018e074cc6f4b1d70b38e52464a38
Merge "Updated README to reflect latest changes."
Merge "Use install_venv_common.py from oslo."