Add support for Remote Configs

[twometresteve]

Goal

Adds support for testing of our Remote Config SDK feature.

Design

Adds a new /error-config endpoint and supporting Cucumber steps for checking requests and managing the responses.

As part of this change I'm forming a new pattern for writing steps, into which we can steadily refactor existing step definitions:

• The step definitions in file under lib/features/steps contain the absolute minimum Ruby code.
• Code files under lib/features/support contain all logic of any complexity

This will allow us to more thoroughly unit test step definitions when appropriate, as well as separating Cucumber-specific aspects from general logic. This could open the door for allowing test frameworks other than Cucumber.

Documentation

Markdown docs added in a new features folder (which I've moved the existing Commands.md into).

Changeset

• error-config endpoint added, backed by the ErrorConfigServlet.
• New Cucumber steps and step definition code in supporting files.
• Existing diagnostic routines updated to include logging of received error config requests.

Tests

Added a new e2e test to exercise the endpoint and new steps. Some of the existing tests were refactored a little to make way for making GET requests in the tests rather than POST.