Make usable where there's no KMS

[rclark]

To be able to keep dependent templates which ImportValue consistent across regions, we need to be able to use this template in any region, even if KMS doesn't exist. In this case this library should export a "fake" KMS value.

To check:

• can we deploy a template that has KMS resources in it, in a region without KMS, as long as the KMS resources are conditionally excluded?
• can we deploy a template with no resources? (pretty sure we can't)

cc @zmully