Reviewer Andy: Extra tweaks to AMI creation

4 changes:
- use m1.medium (not small) for the transient AIO node that gets snapshot'd
- use a 5sec tick on AMI creation (as 1 sec is daft for something that takes minutes)
- set the agreed AMI description text so it doesn't need overriding at a later point.
- force the value of the signup key in AMI case
Tested by running up an AMI and observing desired behaviour.