Support `docker service create --mount` syntax for Swarm `service.create`

[dzimine]

Docker service create offers new, reacher syntax for defining mounts and volumes. But client.services.create only takes the "old" docker run syntax for volumes, which, btw, is not supported by docker service create command. Makes it confusing as we often experiment with CLI and it's a bummer when some options don't translate.

Let's fix the confusion and offer the new syntax alongside with the old one.

PR #1396 ready for review #1396.