[SPARK-2823][GraphX]fix GraphX EdgeRDD zipPartitions

If the users set “spark.default.parallelism” and the value is different with the EdgeRDD partition number, GraphX jobs will throw:
java.lang.IllegalArgumentException: Can't zip RDDs with unequal numbers of partitions

Author: luluorta <luluorta@gmail.com>

Closes #1763 from luluorta/fix-graph-zip and squashes the following commits:

8338961 [luluorta] fix GraphX EdgeRDD zipPartitions
Add unit test and explanation of fix