WIP- Merging back my ansible-mesos fork.

I forked ansible-mesos and https://github.com/mhamrah/ansible-marathon some time ago, and I wanted to get these branches back in sync so we can team up on this role. This PR is to give you an FYI and receive feedback for the merge. I noticed @kienpham2000 was making similar changes, and figured why duplicate. 

Major changes:

* Version 0.20.1 of mesos
* Containerizer is docker,mesos by default
* Provide a deconstructed zookeeper/mesos master url so a playbook can easily create a master cluster based on ```groupvars```
* RedHat family support (issues with zookeeper installation)
* Sourced environment variables for settings, as per Mesos documentation.

Let me know what you think.
I actually have docker_support flag in the upstream repo already:
https://github.com/AnsibleShipyard/ansible-mesos/blob/cd0e5dd550d602ff7875623301dfaa288d421107/tasks/slave.yml#L9

If you think this is better, then we should remove docker_support flag. Also if people want to use docker, this PR doesn't install the docker for them? I think it should since that's what people would expect. I'm currently doing it already at the upstream level. Also please pull the latest upstream into your PR to resolve conflict. I can't merge it now.