graphdriver: promote overlay driver to first

Upgrade overlayfs to first place, now that this will not break driver
usage on existing installs.

Signed-off-by: Vincent Batts <vbatts@redhat.com>

daemon/graphdriver/driver.go

@@ -36,10 +36,10 @@ var (
 	drivers map[string]InitFunc
 	// Slice of drivers that should be used in an order
 	priority = []string{
+		"overlay",
 		"aufs",
 		"btrfs",
 		"devicemapper",
-		"overlay",
 		"vfs",
 	}