Add snapshot plugin type

Update existing snapshot drivers to register as plugins.
Load snapshot driver at containerd startup.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

cmd/containerd/builtins.go

@@ -5,4 +5,6 @@ import (
 	_ "github.com/docker/containerd/linux"
 	_ "github.com/docker/containerd/services/content"
 	_ "github.com/docker/containerd/services/execution"
+	_ "github.com/docker/containerd/snapshot/btrfs"
+	_ "github.com/docker/containerd/snapshot/overlay"
 )