CreateImageWizard: target env validation

Validation is not enforced on the image output step
when selecting and then deselecting an image type. This
is because the only validation which is set is `Required`.
However, after deselecting an image type the input becomes
a key-value object with all the values set to false. This
commit adds a custom validator to check that at least one
value in the object is set to true. Fixes #575

src/Components/CreateImageWizard/CreateImageWizard.js

@@ -24,6 +24,7 @@ import {
 
 import {
     fileSystemConfigurationValidator,
+    targetEnvironmentValidator,
 } from './validators';
 
 const handleKeyDown = (e, handleClose) => {
@@ -433,7 +434,7 @@ const CreateImageWizard = () => {
                 });
         } }
         defaultArch="x86_64"
-        customValidatorMapper={ { fileSystemConfigurationValidator, } }
+        customValidatorMapper={ { fileSystemConfigurationValidator, targetEnvironmentValidator } }
         schema={ {
             fields: [
                 {