CDK-668: Update maven plugin to setup DefaultConfiguration.

Previously, properties set in the plugin's configuration were local and
the getDatasetRepository method had to use repository builders to use
the correct file system. This updates the method to create repository
URIs instead and changes the getConf method to update the default
Configuration.
We could add one, but this seems more direct. It's the same reasoning behind transitioning from a "dataset" query argument to using the path, right? It's shorter if we use the path here and it's unambiguous:
* `repo:hive` creates managed tables
* `repo:hive:/path/to/root` creates external tables in the specified root