config: Generate error copy from valid types map

Renders as:

```
Variable 'invalid_type' type must be one of [string, map, list] - 'not_a_type' is not a valid type
```

config/config.go

@@ -1017,7 +1017,16 @@ func (v *Variable) ValidateTypeAndDefault() error {
 	// If an explicit type is declared, ensure it is valid
 	if v.DeclaredType != "" {
 		if _, ok := typeStringMap[v.DeclaredType]; !ok {
-			return fmt.Errorf("Variable '%s' must be of type string, map, or list - '%s' is not a valid type", v.Name, v.DeclaredType)
+			validTypes := []string{}
+			for k := range typeStringMap {
+				validTypes = append(validTypes, k)
+			}
+			return fmt.Errorf(
+				"Variable '%s' type must be one of [%s] - '%s' is not a valid type",
+				v.Name,
+				strings.Join(validTypes, ", "),
+				v.DeclaredType,
+			)
 		}
 	}
 

config/loader_test.go

@@ -684,7 +684,7 @@ func TestLoadFile_badVariableTypes(t *testing.T) {
 	}
 
 	errorStr := err.Error()
-	if !strings.Contains(errorStr, "'bad_type' must be of type string") {
+	if !strings.Contains(errorStr, "'bad_type' type must be one of") {
 		t.Fatalf("bad: expected error has wrong text: %s", errorStr)
 	}
 }