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.

Change-Id: Ibd1d4ea17c24923ed939357ada95b781e3179cfd