Support ClusterVolumeSpec in the create_volume api

[jonaslb]

Docker's VolumeCreate API takes a ClusterVolumeSpec parameter that it'd be nice to be able to use.

Most commonly, ClusterVolumeSpec.AccessMode.{Scope,Sharing} will need to be set and the Docker CLI also has arguments to set these. But probably the whole ClusterVolumeSpec should be exposed in Python. For example in my case, I'd like to be able to set ClusterVolumeSpec.AccessMode.MountVolume.MountFlags (and I turned to the Python API because I could find no way to do this with the CLI...).

Currently the only workaround I'm aware of is to use the API directly instead of using the otherwise nice Python API.