[SPARK-6132][HOTFIX] ContextCleaner InterruptedException should be quiet

If the cleaner is stopped, we shouldn't print a huge stack trace when the cleaner thread is interrupted because we purposefully did this.

Author: Andrew Or <andrew@databricks.com>

Closes #4882 from andrewor14/cleaner-interrupt and squashes the following commits:

8652120 [Andrew Or] Just a hot fix

core/src/main/scala/org/apache/spark/ContextCleaner.scala

@@ -168,6 +168,7 @@ private[spark] class ContextCleaner(sc: SparkContext) extends Logging {
           }
         }
       } catch {
+        case ie: InterruptedException if stopped => // ignore
         case e: Exception => logError("Error in cleaning thread", e)
       }
     }