Roles, name, README updates, and more

Hi @navidpaya,

We're seeing more requests for a Scout Ansible Playbook, so I went ahead and ran w/what you got started. Some things I added:

* roles
* name (for friendlier names in the scout ui for hosts)
* updated the readme and explained each of the variables

Some things I dropped:

* you don't actually have to run "scout install". I removed this step. If scoutapp.com was unreachable during the playbook run, this step would fail. 
* dropped the `--hostname` option. I'll add this back at some point. Most folks just want a friendlier identifier (where the name comes in).
* added a check to ensure the `scout_key` is provided. I made this null instead of a placeholder.

We're happy to maintain things going forward - I just wanted to give you a heads up.