Import install_venv from oslo

The new version of install_venv_common evaluates requirements.txt
and test-requirements.txt at the same time and we avoid Django 1.6
is installed when evaluating django-nose dependencies.

Change-Id: I2ab7108b3615ac6c0525089b81b80e5c0a4793fc
Closes-Bug: #1250581
(cherry picked from commit 7a0581bb701dbb4d7d9fb1bbe1d7644e98ac2b2e)
Merge "Import install_venv from oslo" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/horizon into stable/havana_routerrules