root-disk constraint broken on ec2

From a user:

> Trying to deploy a charm with some extra root disk space. When using the root-disk constraint defined above I get the following error:
>
> '(error: no instance types in us-east-1 matching constraints "cpu-power=100 root-disk=16384M")'
>
> I’m deploying a bundle with the following constraints: constraints: "mem=4G arch=amd64”, but need more disk-space then the default provided.

It appears Juju is failing to successfully match the specified constraints to an available instance type in ec2 because the instance type entries have RootDisk set to 0 (the root disk size is dynamically allocated when the instance is started).