Use install_venv from oslo to fix no post_process issue

In a recent oslo sync commit Ib9b07fe733dc2c924ac0eb054fa707dd352a812b,
post_process() has been removed from tools/install_venv_common.py.
However, it's still called in tools/install_venv.

The current code for CentOS looks like trying to install the openssl
after trying to compile pyOpenSSL. So this can't be working.

This change switches tools/install_venv.py to use oslo code to align the
effort as well as fixing original no 'post_process' issue.

Note: openstack-common.conf is not updated. Because the project name
variable in install_venv has to be modified after syncing from oslo.
That makes it not an exact sync but modification after sync now.

Change-Id: Ia50075f9b317315eb939ddb66645987bb9ec69c2
Closes-Bug: #1263940
Merge "Use install_venv from oslo to fix no post_process issue"