feat(running-coreos): split vagrant clustering and single machine

The idea behind this change is to:
* Decrease confusion by splitting clustering and single machine instructions.
* To lead with clustering since that's the most powerful intro to CoreOS

Here's a few screenshots of the sections that don't render in markdown:

Cluster channel:
![image](https://cloud.githubusercontent.com/assets/755540/3164275/964d3c6c-eb48-11e3-9cff-040a613293ed.png)

Single machine channel:
![image](https://cloud.githubusercontent.com/assets/755540/3164281/9e0402ec-eb48-11e3-9744-8bffa9988d3d.png)