[SPARK-7775] YARN AM negative sleep exception

```
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.slf4j.impl.Log4jLoggerFactory]
Exception in thread "Reporter" java.lang.IllegalArgumentException: timeout value is negative
  at java.lang.Thread.sleep(Native Method)
  at org.apache.spark.deploy.yarn.ApplicationMaster$$anon$1.run(ApplicationMaster.scala:356)
```
This kills the reporter thread. This is caused by apache#6082 (merged into master branch only).

Author: Andrew Or <andrew@databricks.com>

Closes apache#6305 from andrewor14/yarn-negative-sleep and squashes the following commits:

b970770 [Andrew Or] Use existing cap
56d6e5e [Andrew Or] Avoid negative sleep

yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala

@@ -345,7 +345,7 @@ private[spark] class ApplicationMaster(
               if (numPendingAllocate > 0) {
                 val currentAllocationInterval =
                   math.min(heartbeatInterval, nextAllocationInterval)
-                nextAllocationInterval *= 2
+                nextAllocationInterval = currentAllocationInterval * 2 // avoid overflow
                 currentAllocationInterval
               } else {
                 nextAllocationInterval = initialAllocationInterval