change Swarm dockercoins+healthcheck.yml to be override

[Bret Fisher]

A note for me to later do this:

Update healthchecks.md to use the 18.03+ feature of multiple compose files in a deploy. No need for docker-compose config or redundant lines across two files. Remove unneeded data from dockercoins+healthcheck.yml. Then we could run:

docker stack deploy -c dockercoins.yml -c dockercoins+healthcheck.yml dockercoins in slides to show this feature off while we're updating a service with a healthcheck.