Use Formula#system to install Python packages

Resolve the issue elaborated in #34682 by using Formula#system in Language::Python.setup_install, so that failures in setup.py raise exceptions that interrupt the installation process and so that builds are logged.

I took the approach of making Formula#system public and passing a formula object into setup_install. Jack discussed a couple of other options at https://github.com/Homebrew/homebrew/pull/34682#issuecomment-65716527.

Ansible is updated as an illustration; there's about 14 calls to setup_install in core formulas so far.

Unfortunately this makes our ohais during the installation really ugly; they come out like:

```
==> Downloading https://pypi.python.org/packages/source/p/python-keyczar/python-keyczar-0.71c.tar.gz
Already downloaded: /Library/Caches/Homebrew/ansible--python-keyczar-0.71c.tar.gz
==> python -c import setuptools, tokenize
__file__ = 'setup.py'
exec(compile(getattr(tokenize, 'open', open)(__file__).read()
  .replace('\r\n', '\n'), __file__, 'exec'))
 install --sing
```

so I'll try excluding the shim from pretty_args.
Added a warning.