Fix Spark Dispatcher's image config

[skonto]

The property service.docker-image in config.json is not used. This is a nice property which helps people use a spark image other than the one provided already by Mesosphere.
Tested by deriving an image:

FROM mesosphere/spark:1.1.0-2.1.1-hadoop-2.6
ADD mesos-cluster-dispatcher.properties /opt/spark/dist/conf/mesos-cluster-dispatcher.properties
WORKDIR /opt/spark/dist

[skonto]

@susanxhuynh @ArtRand pls review.

[susanxhuynh]

LGTM, cc @ArtRand

[ArtRand]

@skonto would it be possible to add a unit test to this? I'm already nervous enough about the poor test coverage in this repo.

[skonto]

@ArtRand sure I will have a look could you point me to where do you usually put such tests?
I don't see any universe tests only spark level tests. How should I test this?

[ArtRand]

@skonto the tests usually go in spark-build/tests/test_spark.py, and any files (like a new Dockerfile) go in spark-build/tests/resources. It may be a little tricky/annoying to build another derived imaged as park of the jenkins job (you'd have to change the web-of-scripts around spark-build/bin/jenkins-package-test.sh). If so maybe we can skip it. What do you think @susanxhuynh?

[susanxhuynh]

Having test coverage is good in general. Building a derived image is tricky: you'd have to generate the Dockerfile on the fly if you wanted to derive from the latest image.

[skonto]

@susanxhuynh @ArtRand should I go through this for such a trivial modification? If I were to do this it makes sense to do it for all the settings there... I think it is too much, we can always improve coverage in another PR, thoughts?

[ArtRand]

LGTM. @susanxhuynh if you're ok with this give it a merge? (just running the tests once more because it's been a while)

[ArtRand]

retest this please