[marathon] service checks and improvements

# What's This Do?

After working on the just-merged etcd plugin for #1235 I wanted to take a look at the Marathon plugin. Looks like the key problem I saw was already fixed in #1240. While I was here I:

* Refactored some of the magic numbers and arrays up into class variables
* Removed unnecessary includes
* Switched to the service check API
* Added the "disk" attribute from the metrics

# Thanks!

I appreciate you taking the time to look at this. If I can do anything to help a merge, let me know!
This compat can be dropped, also as `get_v2_apps` and `get_v2_app_versions` could be refactored in one single function with the endpoint as argument because they are the same.

Also that would be the occasion to drop `timeout_event` and `status_code_event` and inline the service checks directly in the function above. Wdyt?