amp config

[subfuzion]

Closes #1613

This PR adds cli and api support for docker swarm configs. The PR also includes a config example.

Verification

$ echo "foo=bar" | amp config create myconfig -
$ amp config ls
# confirm myconfig is displayed in the list
$ docker service create --name configdemo --config myconfig alpine sleep 1000000
$ docker exec $(docker ps --filter name=configdemo -q) cat /myconfig
# foo=bar

$ docker service rm configdemo
$ amp config rm myconfig

[bquenin]

LGTM