Make packages in virtualenv take precedance.

Duplicate installations of packages in system python environment
will take precedance by default and make unforseeable issues, so
we declare to use packages in virutalenv first and then system packages.

Change-Id: I29a56ffad33dfabec3ea73a84dbdc8e943ca8153
Closes-bug: #1354625
Cherry-pick from review.openstack.org/#/c/113052/
Merge "Make packages in virtualenv take precedance." into dev/experimental