Several updates

(The change set looks huge b/c I edited the template in YAML, then converted back to JSON. Feel free to take it or leave it.)

I changed the following about the template:
- Use canonical ubuntu AMIs; no dependency on unofficial code/AMIs.
- Use HVM SSD instances (t2.small by default)
- Use IAM roles & instance profiles instead of an IAM user with host keys
- Add Availability zones as parameter (not all AZs are usable in all
  AWS accounts)
NB: docker registry v0.7.3 hangs unless you allow access to the bucket, and not just the objects in the bucket. See [this comment](https://github.com/dotcloud/docker-registry/issues/400#issuecomment-49386488)