change max backoff timeout to be compatible with the hardcoded preemption_wait.

Reviewed at https://reviews.apache.org/r/17683/
AURORA-183: Upgrade twitter.common.python to 0.3.0

Use the new "pex" utility from twitter.common.python to bootstrap pants
rather than relying on knowledge of the inner workings of
twitter.common.python APIs. I also took the opportunity to refactor the
common "maybe bootstrap a virtualenv" pattern used with some support
tools to use a consistent build-support/NAME.venv pattern.

Testing Done:
Ran ./pants, ./rbt, ./build-support/pex,
./build-support/release/make-python-sdists from clean and dirty repos.

./pants src/test/python:all

Bugs closed: AURORA-183

Reviewed at https://reviews.apache.org/r/17679/