[SPARK-5691] Fixing wrong data structure lookup for dupe app registratio...

In Master's registerApplication method, it checks if the application had
already registered by examining the addressToWorker hash map. In reality,
it should refer to the addressToApp data structure, as this is what
really tracks which apps have been registered.

Author: mcheah <mcheah@palantir.com>

Closes #4477 from mccheah/spark-5691 and squashes the following commits:

efdc573 [mcheah] [SPARK-5691] Fixing wrong data structure lookup for dupe app registration

core/src/main/scala/org/apache/spark/deploy/master/Master.scala

@@ -671,7 +671,7 @@ private[spark] class Master(
 
   def registerApplication(app: ApplicationInfo): Unit = {
     val appAddress = app.driver.path.address
-    if (addressToWorker.contains(appAddress)) {
+    if (addressToApp.contains(appAddress)) {
       logInfo("Attempted to re-register application at same address: " + appAddress)
       return
     }