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.