promote overlay(2) graphdriver

Now that overlay has matured, using
overlay is a better choice than devicemapper
on loopback devices.

This change promotes overlay in the
priority list. It also adds the
overlay2 graphdriver to the list
because overlay2 (if supported)
should be preferred over overlay.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/graphdriver/driver_linux.go

@@ -56,8 +56,9 @@ var (
 		"aufs",
 		"btrfs",
 		"zfs",
-		"devicemapper",
+		"overlay2",
 		"overlay",
+		"devicemapper",
 		"vfs",
 	}