Re-add install_venv_common to openstack-common.conf

Mistakenly removed it in e0272b0578d2a4f5bd75e68baee1b56f49931266.
Was thrown off by the warning saying module not found, the module is
found on the third try (found in tools).

Change-Id: Iaf5a7ba0259d352cb824b2cdb22291ffb7883efc
Merge "Re-add install_venv_common to openstack-common.conf"