Initial version of /health_check endpoint, with tests.

This should resolve mozilla/mofo_system_configurations#70.
made password for bad username test correct.
Factored out is_basic_http_auth_correct() function from health_check().
Factored out calculate_overall_status() function from health_check().
minor code style fixups
remove unused imports.
Merge pull request #677 from toolness/health-check

Initial version of /health_check endpoint, with tests.