Optional mesos_role configuration and tags

[lhoss]

Required config to use mesos roles in marathon!

ps: We need to change some more marathon configs from the defaults.
For this I propose a generic solution to write simple string $value
to the file `/etc/marathon/conf/$key' (where the user just has to be careful not to overwrite any existing config, by a bad $key setting).

Would this be a good solution?
@ernestas-poskus ?

[ernestas-poskus]

🆗 but would be more elegant if you would use #28 marathon_additional_configs

[lhoss]

I guess 'my mistake' that I didn't wait to get this PR (27) merged before even opening #28 (that feature I just created today afternoon btw).

But anyway I'ld like the above PR get (stash)merged also because it is more stable in case you want to remove that role option again (in which case the additional configs might still need an extension, for ex. to remove config-files where an empty value was set (instead of writing the file empty.. where I wonder what marathon is doing !) )

[ernestas-poskus]

I see, ok thank you for contribution merging this and will release under 0.4.0