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.
we should not put implementation details in doc as it will change and doc will become outdated