Use install_venv from oslo to fix no post_process issue

In a recent oslo sync commit Ieb73d5f799423fa8abf71634c86601ba6d32df2f
post_process() has been removed from tools/install_venv_common.py, which
is still called in tools/install_venv.py

Further investigation shows install_venv.py has some legacy distro
specific code, which should be removed at the time introducing
install_venv_common from oslo.

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

Changes being pulled into in install_venv are:
12bcdb7 - 2013-11-25 12:16:11 +0800 - Remove vim header
b5cca99 - 2013-10-09 21:34:00 +0100 - Remove obsolete
redhat-eventlet.patch
9378d3c - 2013-08-13 18:06:11 +0300 - Move `./run_tests.sh` to Oslo

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. Besides
other openstack projects using install_venv from oslo also don't change
openstack-common.conf. So this patch simply follow the common practice
here.

Change-Id: I78a7563fbbde70cfd98cf1bd7a5a4eec5370d31d
Closes-Bug: #1263940
Merge "Use install_venv from oslo to fix no post_process issue"