Add --use-oas2 command line argument support for standalone Swagger Codegen CLI

[reta]

In the scope of #101, it turned out that Swagger Codegen CLI does not support --use-oas2 command line argument (to generate server stubs using OAS 2.x specification), which this pull request aims to fix.

I have checked the usage of for --use-oas2 for:

[ x ] jaxrs-spec
[ x ] jaxrs-cxf
[ x ] jaxrs-resteasy-eap
[ x ] jaxrs-resteasy
[ x ] jaxrs-jersey
[ x ] jaxrs-cxf-cdi
[ x ] jaxrs-di

Sample command line:

java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate     
    -l jaxrs-jersey    
    -i https://petstore.swagger.io/v2/swagger.json     
    -o samples    
    --use-oas2

@HugoMario please mind taking a look, thanks!

[HugoMario]

looks good to me, thanks @reta

[reta]

Thanks a lot, @HugoMario !