Add a pip-cache element.

The element bind mounts a pip cache inside the image build chroot so
that pip downloads can be reused across image builds.  While similar
in purpose to the PyPi element that sets up a mirror, this element
just allows for a reusable download cache and doesn't require anything
to be setup beforehand.

The pip-cache element is not concurrency safe, and that is indicated
in the README for the element.  An upstream bug was file as well:
https://github.com/pypa/pip/issues/1141

Change-Id: Ibd1d4ea17c24923ed939357ada95b781e3179cfd