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.
what if people define their zk host under [zookeepers] in the inventory file, this will fail. I have a solution to fix this in my branch, will probably merge upstream later: https://github.com/kienpham2000/ansible-zookeeper/blob/master/templates/zoo.cfg.j2#L10