There were some inconsistencies with pip install instructions
thoughout Kolla. We fix those here.

Additionally, we fix the virtualenv to properly use the site-packages
on the host if a library is not available in the venv.

Change-Id: Ib84d48e8826bb96060338b3fa0782620c98794a8
Related-Bug: #1524684
Closes-Bug: #1529434