SPARK-2624 add datanucleus jars to the container in yarn-cluster

If `spark-submit` finds the datanucleus jars, it adds them to the driver's classpath, but does not add it to the container.

This patch modifies the yarn deployment class to copy all `datanucleus-*` jars found in `[spark-home]/libs` to the container.
nit: import should be grouped with other `java.*` imports.