Pull in cfnsignal only for AWS k8s and ecs variants

[arnaldo2792]

What I'd like:
By default, all downstream builders whose image matches aws-* will get cfsignal installed. This isn't ideal because those downstream builders might not include in their variant the APIs required by this feature, which will cause the system to be in a degraded state. We have to narrow down when cfsignal is required by only installing it for the aws-k8s and aws-ecs families. If downstreams require this feature, they can explicitly require it in their variant definition.

Any alternatives you've considered:
Keep cfsignal and the system in a degraded state for downstream builders.