[SPARK-5352][GraphX] Add getPartitionStrategy in Graph

[maropu]

Graph remembers an applied partition strategy in partitionBy() and returns it via getPartitionStrategy().
This is useful in case of the following situation;

val g1 = GraphLoader.edgeListFile(sc, "graph.txt")
val g2 = g1.partitionBy(EdgePartition2D, 2)

// Modify (e.g., add, contract, ...) edges in g2
val newEdges = ...

// Re-build a new graph based on g2
val g3 = Graph(g1.vertices, newEdges)

// Partition edges in a similar way of g2
val g4 = g3.partitionBy(g2.getPartitionStrategy, 2)

[ankurdave]

ok to test

[SparkQA]

Test build #25908 has finished for PR 4138 at commit 9fd03a6.

• This patch fails MiMa tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

ISTM this patch causes no error, so please re-test it.

[ankurdave]

Jenkins, retest this please.

[SparkQA]

Test build #26043 has finished for PR 4138 at commit 9fd03a6.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[ankurdave]

Jenkins, retest this please.

[SparkQA]

Test build #26056 has finished for PR 4138 at commit 9fd03a6.

• This patch fails MiMa tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

@maropu Jenkins is correct, you're modifying a public API here, since you add a method to an abstract class. I think this one has stalled anyway; is there another way to accomplish this or should it be closed?

[maropu]

Sorry but mistook to close, so re-make the PR.
#5549

[SparkQA]

Test build #30455 timed out for PR 4138 at commit f72c058 after a configured wait of 150m.