service/snapshotter: move default to client

In order to enforce strict handling of snapshotter values on the
container object, the defaults have been moved to the client side. This
ensures that we correctly qualify the snapshotter under use when from
the container at the time it was created, rather than possibly losing
the metadata on a change of default.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

snapshotter_default_linux.go

@@ -0,0 +1,8 @@
+package containerd
+
+const (
+	// DefaultSnapshotter will set the default snapshotter for the platform.
+	// This will be based on the client compilation target, so take that into
+	// account when choosing this value.
+	DefaultSnapshotter = "overlayfs"
+)