pass caps into docker-py

This adds support for the following yaml syntax:

    service:
        caps:
            add:
            - SYS_NICE

            drop:
            - NET_RAW

--cap-add and --cap-drop was added to docker:
https://github.com/docker/docker/pull/6968

Also this feature has been added to docker-py:
https://github.com/docker/docker-py/pull/314