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.
This original implementation was wrong; it made an asusmption how your hosts file was arranged. I refactored this as a slight variation of your implementation [here](https://github.com/AnsibleShipyard/ansible-zookeeper/blob/master/defaults/main.yml#L10). It's part of AnsibleShipyard/ansible-zookeeper#5. By using an array of hostnames we can supply a group in a playbook with hostvars and get the proper zookeeper setup. Example: https://github.com/mhamrah/ansible-mesos-playbook/blob/role-refactor/playbook.yml#L30