[csharp] Mustache templates do not distinguish generated and native parent classes

Description

The mustache templates do not distinguish generated and native parent classes resulting in invalid code. For example, the override modifier is set in the modelGeneric.mustach template for the ToJson method if a non-array parent exists. But the parent can be a dictionary as well (see Swagger declaration below).

public {{#parent}}{{^isArrayModel}}override {{/isArrayModel}}{{/parent}}{{^parent}}{{#discriminator}}virtual {{/discriminator}}{{/parent}}string ToJson()

Same for the validate method:

{{#parent}}
{{^isArrayModel}}
foreach(var x in BaseValidate(validationContext)) yield return x;
{{/isArrayModel}}
{{/parent}}

BaseValidate does not exist in the dictionary class.

Swagger-codegen version

Version: 2.3.0
Is it a regression? Yes, works well with version 2.3.0-20170713.173236-24.

Swagger declaration file content or url

"KeyValues": {
  "type": "object",
    "additionalProperties": {
      "type": "string"
    }
}

Generates:

public partial class KeyValues : Dictionary<String, string>,  IEquatable<KeyValues>, IValidatableObject

Command line used for generation

java -jar .\swagger-codegen-cli-2.3.0.jar generate -l csharp -i .\swagger.json -c .\config.json -o .\output\ -t .\templates\ --ignore-file-override .swagger-codegen-ignore

Steps to reproduce

1. Create Swagger declaration with the definition mentioned above
2. Create the code java -jar .\swagger-codegen-cli-2.3.0.jar generate -l csharp -i .\swagger.json
3. Try to build the generated code. You will get an compiler error CS0115:

'KeyValues.ToJson()' : no suitable method found to override

Suggest a fix/enhancement

If there would be a template variable like isArrayModel but for dictionaries or native parent classes in general, we could adjust the existing templates easily.

For example like this:

{{#parent}}
{{^isArrayModel}}
{{^isDictionary}}
foreach(var x in BaseValidate(validationContext)) yield return x;
{{/isDictionary}}
{{/isArrayModel}}
{{/parent}}

Start with the C# template modelGeneric.mustach and trace how parent and isArrayModel are provided for dictionary models. Reproduce the KeyValues declaration from the issue, generate the C# code, and compile it to confirm the CS0115 error. Done means dictionary models no longer emit invalid override or BaseValidate code.