[SPARK-13492][MESOS] Configurable Mesos framework webui URL.

## What changes were proposed in this pull request?

Previously the Mesos framework webui URL was being derived only from the Spark UI address leaving no possibility to configure it. This commit makes it configurable. If unset it falls back to the previous behavior.

Motivation:
This change is necessary in order to be able to install Spark on DCOS and to be able to give it a custom service link. The configured `webui_url` is configured to point to a reverse proxy in the DCOS environment.

## How was this patch tested?

Locally, using unit tests and on DCOS testing and stable revision.

Author: Sergiusz Urbaniak <sur@mesosphere.io>

Closes #11369 from s-urbaniak/sur-webui-url.

1 files changed, 16 insertions, 0 deletions

diff --git a/docs/running-on-mesos.md b/docs/running-on-mesos.md
index 912a010812..3a832de95f 100644
--- a/docs/running-on-mesos.md
+++ b/docs/running-on-mesos.md
@@ -390,6 +390,22 @@ See the [configuration page](configuration.html) for information on Spark config
     </ul>
   </td>
 </tr>
+<tr>
+  <td><code>spark.mesos.driver.webui.url</code></td>
+  <td><code>(none)</code></td>
+  <td>
+    Set the Spark Mesos driver webui_url for interacting with the framework.
+    If unset it will point to Spark's internal web UI.
+  </td>
+</tr>
+<tr>
+  <td><code>spark.mesos.dispatcher.webui.url</code></td>
+  <td><code>(none)</code></td>
+  <td>
+    Set the Spark Mesos dispatcher webui_url for interacting with the framework.
+    If unset it will point to Spark's internal web UI.
+  </td>
+</tr>
 </table>
 
 # Troubleshooting and Debugging