RPC: set jetty plugin to latest stable jetty7

- Use latest jetty7 maven plugin.
- Fixed position of commented DB connectors in pom.xml.
RPC: updated jetty execution statement in comments 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.
RPC: use jetty 8 with AJP port configured

- For local testing of Perun we can use Jetty8 with AJP port configured,
  which makes Apache only component, that requires own install and config.
  Of course you must have SSH tunnel to devel DB or your own DB.
Merge pull request #365 from zlamalp/rpc

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