add mount type for volume_binds generated mounts

[kreuzert]

See #536 for more information

[minrk]

I think this might be fixed by #535 which lets you pass through all mount arguments without needing any new config fields. Does that work for you?

[kreuzert]

That's looking interesting. This might fix the issue for persistent user storage volumes. But I'm not sure it will solve the issue for "internal-ssl-certs" volumes. They're not configured previously by the config, so I cannot configure the required mount arguments.

Or am I missing an option to mount the ssl volume with different arguments?

[minrk]

right, that makes sense. I wonder if there's a better passthrough here, because in general I want to avoid exposing underlying options one at a time if it's avoidable, and instead let you passthrough things directly to the docker API.

Do you think there's perhaps an equivalent to #535 for swarm mounts? Does it work to specify type via volume_driver_options and remove type from the Mount kwargs?

Re-reading a bit, I think maybe the type='bind' is actually just wrong for swarm, though it works in single-node tests.