Merge branch 'stable-2.13'

* stable-2.13:
  Make the hostname mandatory in the configuration
  Gracefully handle invalid port value in config
  Make the reporting rate configurable
  Shorten the "Reporting to Graphite... " info log message
  Extract config keys and defaults to constants

Change-Id: I48b6fb7a9e67d478c2c3ef34802d504b405b1d5c