Ability to query policies when creating Docker Volume

[lamw]

VI Admins can create VSAN policies which then are consumed by end users (Developers) when creating the Docker Volumes. However, it doesn't look like Docker can query the plugin capabilities today which would mean the VI Admin must then communicate this info to all the Developers which isn't really a scalable solution.

[msterin]

As a poor man solution, and given Docker is missing any APIs for communicating extra info,
I suggest to report list of avail policies on command line failure:
docker volume create --driver=vmdk --name=doesnotmatter -o policy=some-bad-name
and we'll reply with Error: no such policy: some-bad-name. Available policies are : <list...>