Mandrel root can be specified via environment variable

[ethanrowe]

Mandrel, by design, is heavy handed in its expectations when you invoke its bootstrap machinery.

However, there are cases where it's nice if you don't have that heavy-handedness, and could instead just tell it where the project root lives.

So let's impose an environment variable: MANDREL_ROOT.

If given, then bootstrapping should automatically treat that as the root.
In so doing, it should process a Mandrel.py file if one exists in the specified root.
If no such file exists, that should not cause an error; the default search path and defautl logging behavior should all work fine.