[SPARK-1812] remove default args to overloaded methods

Not supported in Scala 2.11. Split them into separate methods instead.
Hey, I think these changes are unnecessary. 

```diff
@@ -95,10 +95,17 @@ class StreamingContext private[streaming] (
   /**
    * Recreate a StreamingContext from a checkpoint file.
    * @param path Path to the directory that was specified as the checkpoint directory
+   */
+  def this(path: String) =
+    this(null, CheckpointReader.read(path, new SparkConf(), new Configuration).get, null)
+
+  /**
+   * Recreate a StreamingContext from a checkpoint file.
+   * @param path Path to the directory that was specified as the checkpoint directory
    * @param hadoopConf Optional, configuration object if necessary for reading from
    *                   HDFS compatible filesystems
    */
-  def this(path: String, hadoopConf: Configuration = new Configuration) =
+  def this(path: String, hadoopConf: Configuration) =
     this(null, CheckpointReader.read(path, new SparkConf(), hadoopConf).get, null)
 
   if (sc_ == null && cp_ == null) {
```

I was able to compile w/o errors with just above patch for `StreamingContext`.