CORE, RPC: added path to log4j in web.xml

CORE:

- Added log4j.xml since xml format is preferred for new
  versions of software. Since we use custom log4j config
  location in tomcat, this file including log4j.properties
  are ignored. They are probably used when running tests or
  when outside perun-rpc context.
  XML version of log4j is usually loaded by default and then
  properties version is ignored even when found in classpath.

RPC:

- Added path to log4j configuration in web.xml. Base path is replacable
  during maven build by ${perun.conf} property and must be absolute path
  starting with "/".

- Changed expected filename from log4j.properties to log4j.xml !!
  So update your files acordingly on perun instances before
  using this.

  Existing .properties file can be easily converted to .xml format
  using this online tool: http://log4j-props2xml.appspot.com/