[maven-release-plugin] prepare release 1.6.0
Merge tag '1.6.0' into champlain. Merged with '-s ours' to avoid upstream pom changes

Apache Accumulo 1.6.0
Restart the HDP changes file again to track what extra we included.
Adding a license header to silence the rat check failure
Configure apache-rat-plugin to ignore temporary build directories
Move the configuration out of plugins and into pluginManagement to allow children modules to add to excludes
Adding distributionManagement tag in pom.xml