Adds shim layer for writing volume drivers

[cpuguy83]

This allows a plugin writer to not care about the plugin API and rather
focus on the volume driver API from engine.

As an example, engine's built-in local driver can operate as a plugin
with no extra effort with this shim layer.

[cpuguy83]

Just throwing this out there. Had the idea while reviewing the graphdriver PR... which itself has simpler types.
This would require importing github.com/docker/docker/volume for access to volume.Driver and volume.Volume.
We can get rid of the need for volume.Driver, but would still require volume.Volume.

[cpuguy83]

ping @calavera

[calavera]

LGTM

[runcom]

LGTM

[h0tbird]

Be aware of that:

# github.com/docker/go-plugins-helpers/volume
../../go/src/github.com/docker/go-plugins-helpers/volume/shim.go:20: too many arguments in call to d.d.Create
../../go/src/github.com/docker/go-plugins-helpers/volume/shim.go:29: d.d.List undefined (type volume.Driver has no field or method List)
../../go/src/github.com/docker/go-plugins-helpers/volume/shim.go:49: d.d.Get undefined (type volume.Driver has no field or method Get)
../../go/src/github.com/docker/go-plugins-helpers/volume/shim.go:63: d.d.Get undefined (type volume.Driver has no field or method Get)
../../go/src/github.com/docker/go-plugins-helpers/volume/shim.go:76: d.d.Get undefined (type volume.Driver has no field or method Get)
../../go/src/github.com/docker/go-plugins-helpers/volume/shim.go:87: d.d.Get undefined (type volume.Driver has no field or method Get)
../../go/src/github.com/docker/go-plugins-helpers/volume/shim.go:102: d.d.Get undefined (type volume.Driver has no field or method Get)

[cpuguy83]

@h0tbird Do you have your vendor of Docker up to date?

[h0tbird]

I am running on an updated Arch Linux with Docker:

# docker version
Client:
 Version:      1.10.1
 API version:  1.22
 Go version:   go1.5.3
 Git commit:   9e83765
 Built:        Fri Feb 12 00:52:02 2016
 OS/Arch:      linux/amd64

Server:
 Version:      1.10.1
 API version:  1.22
 Go version:   go1.5.3
 Git commit:   9e83765
 Built:        Fri Feb 12 00:52:02 2016
 OS/Arch:      linux/amd64

[cpuguy83]

@h0tbird The docker source code needs to be using the 1.10.1 tag. It seems you've got a different versin checked out.

[h0tbird]

@cpuguy83 Thank you. I see my error, it works now. Sorry for the noise.