RPC: update for jetty plugin for local running of Perun

- Use latest jetty7 maven plugin.
- Fixed position of commented DB connectors in pom.xml.
- If you run perun locally via jetty, you must use: mvn jetty:run-exploded
  in order to run compiled sources, otherwise configuration is taken from
  your sources and no maven filtering is performed => you end up with
  an error about unresolvable ${perun.conf} property for log4j.