Don't hard code SELinux labels into code

These labels can be changed based on installed policy.

I am working on allowing containers within containers, but this
will require different SELinux label types other then container_file_t.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

drivers/overlay/overlay.go

@@ -47,8 +47,7 @@ var (
 )
 
 const (
-	defaultPerms     = os.FileMode(0555)
-	selinuxLabelTest = "system_u:object_r:container_file_t:s0"
+	defaultPerms = os.FileMode(0555)
 )
 
 // This backend uses the overlay union filesystem for containers
@@ -657,6 +656,8 @@ func SupportsNativeOverlay(home, runhome string) (bool, error) {
 func supportsOverlay(home string, homeMagic graphdriver.FsMagic, rootUID, rootGID int) (supportsDType bool, err error) {
 	// We can try to modprobe overlay first
 
+	selinuxLabelTest := selinux.PrivContainerMountLabel()
+
 	exec.Command("modprobe", "overlay").Run()
 
 	logLevel := logrus.ErrorLevel