Add support for environment variables

[gf-atebbe]

This pull request addresses the following two issues

• Issue Mandrel root can be specified via environment variable #8: Allow mandrel root to be specified via an environment variable
• Issue bootstrap basename can be specified via environment variable #13: Allow bootstrap basename to be specified via an environment variable

[ethanrowe]

I'm happy to see tests, but this changes too much core stuff for a single test case to cover.

Given that you're introducing two changes (the mandrel root variable; the bootstrap basename variable), the cases to consider are considerably more complicated:

• Base case: neither variable is defined (the original behaviors)
• Root is specified, bootstrapper is not, Mandrel.py is present in specified root (should parse the file it finds)
• Root is specified, bootstrapper is not, Mandrel.py is not present in specified root (should get basic defaults)
• Root is specified, bootstrapper is specified, the specified file exists in the specified root
• Root is specified, bootstrapper is specified, the specified file does not exist in the specified root
• Root is not specified, bootstrapper is specified, the specified file exists; it should be used and the root should be where that guy was found. This would need testing at various file system levels.
• Root is not specified, bootstrapper is specified, the specified file does not exist within the file system hierarchy; this should blow up, just like it does now if Mandrel.py can't be found.

So there's quite a few test cases that would need to be covered.

It's considerably simpler if only one of the issues is done: either specifying a root, or specifying a basename, but not both.

But as it stands, the test only covers the case where both are set, it doesn't treat them separately.

[gf-atebbe]

@ethanrowe Apologies for the extremely slow turn-around on this. I added a bunch of tests to the PR to address the scenarios you outlined above.