SPARK-2277: clear host->rack info properly

Hi mridulm, I just think of this issue of [#1212](https://github.com/apache/spark/pull/1212): I added FakeRackUtil to hold the host -> rack mapping. It should be cleaned up after use so that it won't mess up with test cases others may add later.
Really sorry about this.

Author: Rui Li <rui.li@intel.com>

Closes #1454 from lirui-intel/SPARK-2277-fix-UT and squashes the following commits:

f8ea25c [Rui Li] SPARK-2277: clear host->rack info properly

core/src/test/scala/org/apache/spark/scheduler/TaskSetManagerSuite.scala

@@ -475,6 +475,7 @@ class TaskSetManagerSuite extends FunSuite with LocalSparkContext with Logging {
     // Valid locality should contain PROCESS_LOCAL, NODE_LOCAL, RACK_LOCAL and ANY
     assert(manager.myLocalityLevels.sameElements(
       Array(PROCESS_LOCAL, NODE_LOCAL, RACK_LOCAL, ANY)))
+    FakeRackUtil.cleanUp()
   }
 
   test("test RACK_LOCAL tasks") {
@@ -505,6 +506,7 @@ class TaskSetManagerSuite extends FunSuite with LocalSparkContext with Logging {
     // Offer host2
     // Task 1 can be scheduled with RACK_LOCAL
     assert(manager.resourceOffer("execB", "host2", RACK_LOCAL).get.index === 1)
+    FakeRackUtil.cleanUp()
   }
 
   test("do not emit warning when serialized task is small") {