Add Checkstyle rule to enforce the order of Java imports

There are Eclipse and Idea prererences for import order in dev tools
https://github.com/hawkular/hawkular-build-tools/blob/master/ide-configs/eclipse/hawkular-eclipse-preferences-java-codestyle-organizeimports.importorder

The settings should be enforced by Checkstyle too, probably using this rule:
http://checkstyle.sourceforge.net/config_imports.html#ImportOrder