Properly use strings as values for enums in setting overrides

This is interpreted as a Python function, so if it doesn't have those quotes it'll be interpreted as a variable name, and that variable doesn't exist.

Contributes to issue CURA-8627.

resources/definitions/ultimaker2.def.json

@@ -91,13 +91,13 @@
             "value": false
         },
         "retraction_combing": {
-            "value": "no_outer_surfaces"
+            "value": "'no_outer_surfaces'"
         },
         "skin_monotonic" : {
             "value": true
         },
         "top_bottom_pattern" : {
-            "value":  "zigzag"
+            "value":  "'zigzag'"
         }
     }
 }