docs(installing_deis): import AWS provisioning documentation

Also includes some nice doc cleanup between the DigitalOcean and the AWS provisioning guides.

This sentence used to be where we pointed them back to the README, but I'm not sure what we mean by it now since our general intention is to do the opposite. I say we either strike this sentence or replace it with:
```
Please refer to the scripts in `contrib/ec2`_ while following this documentation.
```