Move responsibility of ls/inspect to volume driver

Makes `docker volume ls` and `docker volume inspect` ask the volume
drivers rather than only using what is cached locally.

Previously in order to use a volume from an external driver, one would
either have to use `docker volume create` or have a container that is
already using that volume for it to be visible to the other volume
API's.

For keeping uniqueness of volume names in the daemon, names are bound to
a driver on a first come first serve basis. If two drivers have a volume
with the same name, the first one is chosen, and a warning is logged
about the second one.

Adds 2 new methods to the plugin API, `List` and `Get`.
If a plugin does not implement these endpoints, a user will not be able
to find the specified volumes as well requests go through the drivers.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/mounts.go

@@ -11,10 +11,11 @@ import (
 func (daemon *Daemon) prepareMountPoints(container *container.Container) error {
 	for _, config := range container.MountPoints {
 		if len(config.Driver) > 0 {
-			v, err := daemon.createVolume(config.Name, config.Driver, nil)
+			v, err := daemon.volumes.GetWithRef(config.Name, config.Driver, container.ID)
 			if err != nil {
 				return err
 			}
+
 			config.Volume = v
 		}
 	}
@@ -27,10 +28,10 @@ func (daemon *Daemon) removeMountPoints(container *container.Container, rm bool)
 		if m.Volume == nil {
 			continue
 		}
-		daemon.volumes.Decrement(m.Volume)
+		daemon.volumes.Dereference(m.Volume, container.ID)
 		if rm {
 			err := daemon.volumes.Remove(m.Volume)
-			// ErrVolumeInUse is ignored because having this
+			// Ignore volume in use errors because having this
 			// volume being referenced by other container is
 			// not an error, but an implementation detail.
 			// This prevents docker from logging "ERROR: Volume in use"