Sync latest install_venv_common from oslo.

This commit syncs in the latest version of install_venv_common
from oslo, which fixes the import issue that was previously
addressed by change: I8a3df02a51c2c777d8f47cb8bc72d0644ae47095
which gets reverted by this commit.

Change-Id: I910610da5a4c413711819ca39627cb9ee94814ef
Merge "Sync latest install_venv_common from oslo."