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.
There are two patterns for external URIs, for backward-compatibility. The original URIs were `dataset:hive:/datasets/ns/dataset` to match the HDFS URIs (just replace "hive" with "hdfs"). The second URI doesn't use an external repository, it uses the managed repository where there is no root location. In the long term, we should remove the distinction between external and managed repositories because both will create external tables when a location for the dataset is passed in, as of the latest changes. This was done to enable using the second URI form (minus location) in all cases other than dataset creation. We need to continue supporting the external repository only because it was the old way of creating external datasets. I think we should deprecate it for 1.0.