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/steps/imageOutput.js

@@ -50,6 +50,9 @@ export default {
             validate: [
                 {
                     type: validatorTypes.REQUIRED
+                },
+                {
+                    type: 'targetEnvironmentValidator'
                 }
             ],
         }