ISPN-4499 Prevent shadowing user-defined jgroups configuration file

https://issues.jboss.org/browse/ISPN-4499

The solution is following:
* move the defaults files to a specific directory with "normal" name and also rename the files
* log a warning when more than 1 config with that name is on classpath

NOTES:
I did not place the config files under META-INF as when I was trying this with JDG EAP modules where the application deployed in WildFly refers to infinispan-core module, the configuration files could not be found (not sure, maybe some WildFly classloading magic?) Also default-configs folder looks better to me:)

Regarding defining the order of loading classes, this should be achieved by Util.getClassLoaders which retuns the application class loader as the first one so the application should look at its own resources first. However, for some reason, this does not work correctly when as-modules are used and I'm note sure it would work in OSGi either. So throwing the warning when more config files with the same name are on classpath is a good thing, IMO.

Does this need a change in docs?
We try to change existing ones as well, when we notice them.