[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.