[SPARK-17437] Add uiWebUrl to JavaSparkContext and pyspark.SparkContext

## What changes were proposed in this pull request?

The Scala version of `SparkContext` has a handy field called `uiWebUrl` that tells you which URL the SparkUI spawned by that instance lives at. This is often very useful because the value for `spark.ui.port` in the config is only a suggestion; if that port number is taken by another Spark instance on the same machine, Spark will just keep incrementing the port until it finds a free one. So, on a machine with a lot of running PySpark instances, you often have to start trying all of them one-by-one until you find your application name.

Scala users have a way around this with `uiWebUrl` but Java and Python users do not. This pull request fixes this in the most straightforward way possible, simply propagating this field through the `JavaSparkContext` and into pyspark through the Java gateway.

Please let me know if any additional documentation/testing is needed.

## How was this patch tested?

Existing tests were run to make sure there were no regressions, and a binary distribution was created and tested manually for the correct value of `sc.uiWebPort` in a variety of circumstances.

Author: Adrian Petrescu <apetresc@gmail.com>

Closes #15000 from apetresc/pyspark-uiweburl.

python/pyspark/context.py

@@ -332,6 +332,11 @@ class SparkContext(object):
         """
         return self._jsc.sc().applicationId()
 
+    @property
+    def uiWebUrl(self):
+        """Return the URL of the SparkUI instance started by this SparkContext"""
+        return self._jsc.sc().uiWebUrl().get()
+
     @property
     def startTime(self):
         """Return the epoch time when the Spark Context was started."""