Refactor marathon recipe, add event_subscriber flag

Marathon recipe now uses the same method of mapping chef attributes
from node['marathon']['flags'] to /etc/marathon/conf/* that the
mesos-master and mesos-slave recipes use for their commandline options.

Added --event_subscriber=http_callback for Bamboo's sake

Change-Id: If32e7d89a181a02f5329a383451a86e95bbd6208