Switch almost all python tasks over to use cached chroots.
Review Request #2486 - Created July 15, 2015 and submitted
Now that we're on pex 1.0+, this works. And indeed, the effect is dramatic: On my laptop running all the unittests went from ~5:00 mins to around 1:45 mins. Note that binary_create doesn't get any benefit from this yet, because the pex_info contains timestamps, which makes the cache key miss every time. We can think about a fix for this, and then get rid of temporary_chroot() entirely. Also fixes a few random nits that I noticed on the way.
CI passes (faster) here: https://travis-ci.org/pantsbuild/pants/builds/71142392