This fixes getStorageLevel for EdgeRDDImpl and VertexRDDImpl (and therefore for Graph).

See code example on JIRA which failed before but works with this patch: [https://issues.apache.org/jira/browse/SPARK-5534]
(The added unit tests also failed before but work with this fix.)

Note: I used partitionsRDD, assuming that getStorageLevel will only be called on the driver.

CC: mengxr  (related to LDA PR), rxin  ankurdave   Thanks in advance!

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #4317 from jkbradley/graphx-storagelevel and squashes the following commits:

1c21e49 [Joseph K. Bradley] made graph getStorageLevel test more robust
18d64ca [Joseph K. Bradley] Added tests for getStorageLevel in VertexRDDSuite, EdgeRDDSuite, GraphSuite
17b488b [Joseph K. Bradley] overrode getStorageLevel in Vertex/EdgeRDDImpl to use partitionsRDD