Selective generation doesn't support CSV list

Description

When passing models or apis as a CSV for selective generation only the first part is used. Documentation: https://github.com/swagger-api/swagger-codegen#selective-generation

Swagger-codegen version

2.4.16 (current master branch).

Swagger declaration file content or url

https://petstore.swagger.io/v2/swagger.json

Command line used for generation

Windows 10 command prompt
Java jdk1.8.0_261

java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -i https://petstore.swagger.io/v2/swagger.json -l csharp -o /var/tmp/pet_api_client -D apis=User,Pet -D models=User,Pet -D supportingFiles

Steps to reproduce

If you change the command line system properties other models will be generated.

• -D apis=User,Pet -D models=User,Pet
  Generates only the UserApi.cs and User.cs
• -D apis=Pet,User -D models=Pet,User
  Generates only the PetApi.cs and Pet.cs

I've added some logging to DefaultGenerator.java.

String apiNames = System.getProperty("apis");
LOGGER.info("apiNames " + apiNames);

String modelNames = System.getProperty("models");
LOGGER.info("modelNames " + modelNames);

They always only print the part before the first comma.

Also tested with this:

• -D apis=Us,erPet -D models=Us,erPet
  This prints "Us" both times.

So nothing beyond the first comma works.

Suggest a fix/enhancement

Somehow getting the entire CSV. Not sure if there are alternatives for System.getProperty if that doesn't work.
Lines of code:

https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultGenerator.java#L309

https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultGenerator.java#L461

Start in modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultGenerator.java at the referenced lines around 309 and 461, then reproduce the issue with the petstore specification and comma-separated apis and models options. Done means the complete CSV values are read and both requested API and model entries are generated rather than only the first item.