conftool tox-py27-jessie is now voting

The job run on the Jessie slaves. Giuseppe kindly refactored some of the
contint puppet manifest to install required python materials on the
Jessie machines.

We also upgraded to pip 7.1.0 which caches compiled modules as wheels.

Bug: T103972
Change-Id: Id9bd92e1874a353200365c3a55b0b2979a8e777f