RPC: update for jetty plugin for local running of Perun

- Use jetty 8 maven plugin.
- Fixed position of commented DB connectors in pom.xml. They are probably not needed anyway.
- 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.
- Jetty is configured with AJP 1.3 connector on port 8009 which can consume ENV properties from
  our standard Apache configuration. Non-authz access is not possible anyway, without change in
  source code (http connector is commented in jetty.xml).

So for local running of Perun server behind https with authz you need:

- configured DB or SSH tunel to devel DB
- proper DB/Perun configuration in /etc/perun
- run mvn jetty:run-exploded in perun-rpc folder
- configured Apache with e.g. certificate authz