Add integration test suite

[GUI]

One thing we've needed for a while is a test suite we can run to test the entire API Umbrella software stack, with all the components in place. While we have fairly good test coverage on the gatekeeper project (where most of the custom code lives), we've been lacking formal tests on the integration of the other important components, which includes things like nginx and varnish:

For example, having such an integration test suite would help test bugs like these (and also ensure regressions don't pop up):

• Testing OPTIONS requests with no body: OPTIONS request with no body results in 503 errors when Varnish is enabled NREL/api-umbrella#28
• Testing how caching interacts with the rest of the stack: Re-enable the HTTP caching layer #55
• Testing how analytics logging is performed when different components are or aren't hit: Ensure that lower-level rate limit errors are captured in the analytics system #106

[GUI]

This is one of the sizable tasks that sort of got wrapped up in the effort to work on agency-specific admin privileges and re-enabling the caching layer (but I wanted to split it out, since it had become a distinct beast). Basically, having this kind of test suite will make it much easier for us to tackle these type of tasks that touch multiple components of the stack (and we'll be able to do so with much more confidence moving forward).

The good news is that that I now have this full integration test suite to a point where I'm feeling pretty good about it testing our full stack (and I've discovered some odd edge-cases we're now better handling in the process). There's still definitely areas we could use more testing, but the foundation of the important pieces are there. The primary focus of the integration test suite has been on general proxying behavior, gzip handling, and caching behavior (since these are the super important pieces we don't mess up or unexpectedly change). Some initial integration tests related to rate limits and analytics logging have also been added.

This testing work is currently sitting out out on the config branches (and is tied to the move toward the omnibus installer):
https://github.com/NREL/api-umbrella-router/tree/config
https://github.com/NREL/api-umbrella-gatekeeper/tree/config

This work hasn't made its way to master yet, so it's not quite done, but I'm hoping to tackle some of the last pieces necessary to get those branches merged as part of this milestone.

[GUI]

These integration tests have made their way to master as part of #123