WIP: Merging back my ansible-marathon fork

As per AnsibleShipyard/ansible-mesos#16 I want to get my ansible-* forks back into the upstream master. I noticed some duplicate work and want to combine efforts. This PR is to get these changes on your radar and foster feedback for a merge.

This is similar to upstream with a few caveats:

* It uses a custom haproxy_dns_cfg script I created which will build frontent/backend blocks based on the marathon app name, as per http://blog.michaelhamrah.com/2014/06/setting-up-a-multi-node-mesos-cluster-running-docker-haproxy-and-marathon-with-ansible/.
* Preliminary RHEL support (WIP)
* I deconstructed the mesos_master and zookeeper_state urls to better align with groupvars when running a multi-node playbook. Previously I let that functionality bleed into this role, and now they are discrete parts.
* Cleanup of unused variables
* Marathon 0.7.3 support
* Tweaks to tasks/conf.yml to make the file less verbose (uses ```lineinfile``` where appropriate)