## What changes were proposed in this pull request?

New config var: spark.mesos.docker.containerizer={"mesos","docker" (default)}

This adds support for running docker containers via the Mesos unified containerizer: http://mesos.apache.org/documentation/latest/container-image/

The benefit is losing the dependency on `dockerd`, and all the costs which it incurs.

I've also updated the supported Mesos version to 0.28.2 for support of the required protobufs.

This is blocked on: https://github.com/apache/spark/pull/14167

## How was this patch tested?

- manually testing jobs submitted with both "mesos" and "docker" settings for the new config var.
- spark/mesos integration test suite

Author: Michael Gummelt <mgummelt@mesosphere.io>

Closes #14275 from mgummelt/unified-containerizer.