Bug 952280 - mach python commands now use virtualenv APIs; r=mshal

The Python-related mach commands were written before we had a virtualenv
API exposed to the mach command context. This patch updates those
commands to use the newer APIs. As a bonus, these commands now work
without running configure!

--HG--
extra : rebase_source : ea394d6fc0c5fa2d3a3a6ed25fc59ce6be40690c
extra : amend_source : e841d57a2578c93b778ef73c68c35a8cc7cfde44