[SPARK-15271][MESOS] Allow force pulling executor docker images

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Mesos agents by default will not pull docker images which are cached
locally already. In order to run Spark executors from mutable tags like
`:latest` this commit introduces a Spark setting
(`spark.mesos.executor.docker.forcePullImage`). Setting this flag to
true will tell the Mesos agent to force pull the docker image (default is `false` which is consistent with the previous
implementation and Mesos' default
behaviour).

Author: Philipp Hoffmann <mail@philipphoffmann.de>

Closes #14348 from philipphoffmann/force-pull-image.