ACCUMULO-2651 Add missing execution id to apache-rat-plugin

  The missing execution id on the execution of the apache-rat-plugin
  evidently causes some build problems in some environments, especially
  when the specified execution phase is other than the default. This
  change adds an execution id, and causes the check goal to bind to its
  default phase in the build lifecycle (which is 'validate', instead of
  'verify').
Merge branch '1.6.0-SNAPSHOT'