Custom logging.yml location

Adding the ability to specify an alternate logging configuration file via system property. Documentation updated.

Closes #2044
I am a bit conflicted on the removal of `Files.walkTree` in case you specify a location for the logging file(s). The idea there is that multiple files can be loaded and merged, for instance plugins can provide their own files that are additional to the default one. Currently every file that starts with `logging` gets loaded, not only from within the `config` dir but from within any of its subdir too (although we are restricting this a bit in #7457). 
I'd rather treat the specified location as some kind of pattern (even better a prefix) and leave the walkTree as is, so that other logging files on the tree are still loaded if they match the specified pattern. This should result in a much smaller change.