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)