make Formula#system public

Allows methods outside Formula, like Language::Python.setup_install, to
take advantage of Formula#system's logging and raise-on-failure
behavior.
use Formula#system in Language::Python.setup_install

Require that setup_install accepts a formula object (which may be nil)
in order to invoke Formula#system when appropriate.
ansible: update setup_install calls
hide python setup.py shim in Formula#system
ansible: update 1.8.2 bottle.