Specify the mapfish configuration file (config.yaml) per request (GEOS-4531)

1) additional implemented handleRequestInternal() to handle requests to create.json and info.json ,
2) refactored original method for setting the config directory to setConfig(), which is used both by handleRequestInternal() and setInitParameters()
3) additionally remember the initial config at start
4) remember the actual config in configPropActual to set the config only if really changed