Support Mesos fetcher cache

[turb]

It would be awesome if Chronos supported Mesos fetcher cache introduced in 0.23, see http://mesos.apache.org/documentation/latest/fetcher/

As far as I see:

• the easy part is to set the cache attribute at MesosTaskBuilder.scala:87
• the delicate part is the introduction of this new parameter in Chronos API (and other attributes, by the way)

Marathon is also speaking about it here: mesosphere/marathon#1953

Thanks!

[turb]

FYI this feature has been released in Marathon 0.15, see https://github.com/mesosphere/marathon/releases/tag/v0.15.0

I can work on a PR to have the same API-breaking modification, but I would prefer to have some feedback on its opportunity before doing it.

[djzort]

+1 super useful