Merge branch 'stable-2.15'

* stable-2.15:
  Allow to disable health check endpoint
  AbstractIndexRestApiServlet: Log operation type in lower case
  Make debug logging consistent in forwarder and cache servlet
  Use Logger's built-in string formatting
  Restrict endpoints to change health status to admins
  Add unit tests for HealthServlet
  Initial implementation of health servlet

Change-Id: Idf195be6d9d08e48d10ad0658270da679669a5b6