Find and consolidate hardcoded paths in DE code, and mock for unit tests

[shreyb]

This issue consists of three parts:

• [ ] Find hardcoded paths in DE code (e.g. configuration file directories)
• [ ] Consolidate all of those into a single file, which then gets imported as necessary
• [ ] Mock the file in (2) for unit tests so that they don't fail if we don't, for example, have /etc/decisionengine on a dev machine or test container

This needs to be done for both the decisionengine code and decisionengine_modules code.

[shreyb]

I actually think the first two parts were done in #168. The current design, implemented by that PR, is that various modules import thedecisionengine.framework.config if they need access to the configuration. The submodules within that module contain functions to dynamically find the configuration files, validate them, and load their values into memory. This is the way it should be, IMO, so I see no need to change that.

That means that the bulk of the work here is actually on the testing side, and mocking where that config module looks for configuration files.

What do you think, @mambelli?