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.
If you wanted to create a dataset in a specific directory then the first URI is correct. If you want to open a repository that creates external tables, that is the right URI. You can also use `dataset:hive:/path/to/ns/dataset`. The `dataset:hive:ns/ds` pattern is relative to whatever root Hive chooses to use, which is the warehouse directory. Setting `location` should be seen as overriding that warehouse location. The external repository URI is that way because it isn't a relative URI. It is an absolute URI that points to a particular path where datasets should be located.