Add support for running tox with PyPy and use it for Marconi

This works by installing PyPy from the Ubuntu PPA, and then invoking
`tox -e pypy` (similar to all the other Python buidlers). PyPy shares
nodes with the Python3 builders.

Change-Id: Ibf683c1995b0f5c04c93c56010befb222b386d7a