[req] Purge synced services on exit

Currently, consul-aws publishes the consul services and nodes. But does not clean-up after itself on exit. When we stopped the services and removed the container daemon we found all services were still listed.

I found this out when I decided to remove the discovery zone (via Terraform). AWS Cloud Map will not let one delete the zone until all services are deleted, and one cannot remove a service until all instances are deleted. There was much ... clicking that day.

Thank you

Start by tracing where consul-aws publishes services and nodes and how process exit is handled. Check the AWS Cloud Map cleanup flow and verify that stopping consul-aws removes its services and instances, leaving the discovery zone deletable.

Written by the indexing model from the issue text.