Enables plugins to define default logging configuration for their needs.

- adds support for multiple logging configurations under the config dir (will pick up any logging.xxx in the config folder tree)
- plugins can now define a top level config directory that will be copied under es config dir and will be renamed after the plugin name (same as the support we have the plugin "bin" dirs)

 Closes #6802
This one should extend `ElasticsearchTestCase`, otherwise it will use the junit runner instead of the randomized one.