[SPARK-4142][GraphX] Default numEdgePartitions

Changing the default number of edge partitions to match spark parallelism.

Author: Joseph E. Gonzalez <joseph.e.gonzalez@gmail.com>

Closes #3006 from jegonzal/default_partitions and squashes the following commits:

a9a5c4f [Joseph E. Gonzalez] Changing the default number of edge partitions to match spark parallelism