Develop a scoring system for customer environments

Part of epic gitlab-org&1338 (closed). We have two mechanisms for comparing customer environments to the reference environment - the baseline performance tests Quality runs against the environment with predefined data and predefined endpoints, and the real-world traffic comparisons using fast-stats (See epic for more information).

Now that we have these methods of comparing environments, we need a way to boil that data down into a simplistic environment score. Customers may not understand what the output from our comparisons means so this score will give them a clearer picture of how well their environment performs. We can then back that 'score' up with concrete steps on what to do to improve it (faster network, faster disk).

cc/ @wchandler