Move the daemon/graphdriver package to storage

Even though we refer to them commonly as "storage drivers" and they are used
by more subsystems than just the daemon package - the container, image, and
volume storage library is buried under the daemon package and is called
`graphdriver` which is a very generic name. Also, some types it exposes like
`graphdriver.Driver` is a stutter and is not idiomatic Go.

This change moves the `daemon/graphdriver` package to the top-level of the
`github.com/docker/docker` package tree under the name `storage`. Users of this
package now deal with `storage.Driver` objects! And other subsystems which use
it, like the `graph` package (another unfortunate name), have one fewer thing
they have to reach into the `daemon` package for.

All references to `graphdriver` (with the exception of environment variables
and Go build tags) have been updated to use the new package location.