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.
No, because this is constructing a repository URI. The repository URI defines a default location that will be used to construct datasets if the descriptor doesn't specify one.