Add support for shared module documentation fragments

See https://groups.google.com/forum/#!topic/ansible-devel/B-Md0nYB0s4

This pull request adds support for shared module documentation fragments.

Modules can share documentation, by extending a fragment located in `ansible/utils/module_docs_fragments.py`

The code that handles the merging, is done in `ansible/utils/module_docs.py`

The `notes` section is appended to.  All other sections are looped over and either set if they don't exist in the fragment, or updated.

I've added the fragments for the `rax` modules thus far.  Since I was touching the DOCUMENTATION strings for all `rax` modules, I also made some cleanup there as well.

Sample DOCUMENTATION from a module:

```yaml
---
module: rax_queue
short_description: create / delete a queue in Rackspace Public Cloud
description:
     - creates / deletes a Rackspace Public Cloud queue.
version_added: "1.5"
options:
  name:
    description:
      - Name to give the queue
    default: null
  state:
    description:
      - Indicate desired state of the resource
    choices:
      - present
      - absent
    default: present
author: Christopher H. Laco, Matt Martz
extends_documentation_fragment: RACKSPACE
```

The `extends_documentation_fragment` key references a variable from `ansible/utils/module_docs_fragments.py`.  The provided value is always uppercased.