Test making copy of environment

(Note: I will include tests with my change next time).

Here is a test that confirms the environment is not tainted across `cmd.run` calls.

I used [flexmock](https://github.com/has207/flexmock) for mocking in the test and added a `dev_requirements.txt` file.  In addition to that, this tests feels way against the grain and I [hacked away](https://github.com/rca/salt/tree/strongarm_tests_into_running) to get tests running on my machine.

Please let me know what can be done to make this better.