Solrcas should not resolve the solrmapping file only via the classpath

The solrmapping file is currently only resolved via the classpath. It should be possible to place a solrmapping file next to the AE descriptor and load it like referenced descriptors.

The Solrcas jar file also contains a default solrmapping file. That should not be included in the jar file, otherwise it might accidentally be loaded depending on the location of that jar on the classpath.