Use fixtures for all shell environment variables

Currently only the variables related to the test are set, so
any other OS_* variables in the local environment can affect
the test results leading to confusing results.  Setting all
variables not passed into fake_env to None will delete them
in the fixture, hence ensuring a consistent test environment.

Change-Id: I24a07ef5fc89ca9fff121fe206b8e314f23c4402