Use optparse for install_venv_common

In order to bootstrap a virtualenv from an unmodified 2.6 Python environment,
we need to use optparse instead of argparse or openstack.common.cfg.

Fixes bug 1174926

Change-Id: Iad636c7adf7b184756eeba5ca760d8a76fc0e354
Merge "Use optparse for install_venv_common"