Support configs for Swarm mode services

In Docker 17.06.0 the concept of configs are introduced:

Add support for services to carry arbitrary config objects moby/moby#32336, docker/cli#45, moby/moby#33169

Config endpoints are available in the Docker remote API under /configs.
The implementation and interface is similar to secrets.
- This would cover the ability to create, inspect and remove a config. Updating does not seem to be supported by the API.
- and be able to assign a config to a service, and remove it. Just like the Service Secrets.

Documentation:

I'm happy to do the work on the initial implementation.

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