Copied environment variables test and comment over

Fixes https://clusterhq.atlassian.net/browse/FLOC-1044

If this is suitable, I will add the new dependencies (currently just the latest https://pypi.python.org/pypi/PyMySQL) to setup.py, and create a @skipUnless decorator for each of the dependencies, similar to require_mongo in testtools.py. Each dependency must also go in the internal documentation for packages used.

However, I understand that https://pypi.python.org/pypi/MySQL-python/ may be a more suitable option. I chose PyMySQL because it is pure Python, so on OS X for example it does not require ``brew install mysql``.
Cleanup should happen immediately when the object is first created, otherwise an exception or failed assertion earlier on will mean the `addCleanup` is never reached.