Improve marathon plugin

# 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!