Fix missing function error in install_venv

The previous commit, 3362ee1337d47ab8930407c58311a811d471a7a4,
a sync with Openstack Common, removed all definitions of the
post_process() function from the Distro class and all of its
child classes in install_venv_common.py.

This fix simply removes the call to the function in
install_venv.py

The issue is similar to #1263940 in tempest:
https://bugs.launchpad.net/tempest/+bug/1263940

Change-Id: Iedd0c29789c4b2ab9be880f9bd320b39a9bdd277
Closes-Bug: #1307705
Merge "Fix missing function error in install_venv"