[STORM-3854] Fix maven-pmd-plugin errors (#3472)

* [STORM-3854] Fix maven-pmd-plugin errors

* [STORM-3854] Add @Override reuired by PMD in storm-client

* [STORM-3854] Add @Override reuired by PMD in storm-server

* [STORM-3854] Add @Override notations in external and example

* Dummy commit to trigger Travis build
17 files changed