Rename SupportsSELinux to SELinuxRelabel

The field in fact says that the container runtime should relabel a volume
when running a container with it, it does not say that the volume supports
SELinux. For example, NFS can support SELinux, but we don't want NFS
volumes relabeled, because they can be shared among several Pods.

pkg/volume/testing/testing.go

@@ -686,9 +686,9 @@ func getUniqueVolumeName(spec *Spec) (string, error) {
 
 func (_ *FakeVolume) GetAttributes() Attributes {
 	return Attributes{
-		ReadOnly:        false,
-		Managed:         true,
-		SupportsSELinux: true,
+		ReadOnly:       false,
+		Managed:        true,
+		SELinuxRelabel: true,
 	}
 }