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.
I'm not sure I like using the path to name the dataset. I'd rather our URIs were consistent, ala the comment on CDK-633, but I wanted to start a discussion thread on the mailing list first.