introduce docker stack config

[ndeloof]

- What I did
Many user report interoperability issue trying to pipe docker compose config to docker stack deploy`` Unclear to me why they do so, as stack deploysupports merging multiple compose file and variable substitution... Anyway this PR introducesdocker stack config` to offer equivalent feature relying on stack's compose loader

- How I did it
load model the same way stack deploy does and marshal into yaml

- How to verify it
docker stack config --compose-file compose.yml

- Description for the changelog
introduce docker stack config command

- A picture of a cute animal (not mandatory but encouraged)

[ndeloof]

closing as a duplicate for (better) #3544