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.
In fact, nevermind. These jars need to be in Spark's classpath, not the user's classpath, so `cachedSecondaryJarLinks` wouldn't work for this after my change goes in.