Add IsMigratableToCSI to volume plugin interface

pkg/volume/plugins_test.go

@@ -75,6 +75,10 @@ func (plugin *testPlugins) CanSupport(spec *Spec) bool {
 	return true
 }
 
+func (plugin *testPlugins) IsMigratedToCSI() bool {
+	return false
+}
+
 func (plugin *testPlugins) RequiresRemount() bool {
 	return false
 }