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.

Author: Jim Lim <jim@quixey.com>

Closes #3238 from jimjh/SPARK-2624 and squashes the following commits:

3633071 [Jim Lim] SPARK-2624 update documentation and comments
fe95125 [Jim Lim] SPARK-2624 keep java imports together
6c31fe0 [Jim Lim] SPARK-2624 update documentation
6690fbf [Jim Lim] SPARK-2624 add tests
d28d8e9 [Jim Lim] SPARK-2624 add spark.yarn.datanucleus.dir option
84e6cba [Jim Lim] SPARK-2624 add datanucleus jars to the container in yarn-cluster