Swagger Codegen will generate clients from invalid specs

Description

swagger-codegen generate will generate a client on specs that swagger-codegen validate will raise an uncaught exception for. I noticed this on a version of the 1.6 Kubernetes spec used to generate the Python client, but as of Kubernetes 1.8 the unedited spec is not valid but can still be used to generate a client. Other tools that only operate on valid specs break on the Kubernetes 1.8 spec.

Swagger-codegen version

I tested this with 2.2.3.

Swagger declaration file content or url

The Kubernetes 1.8 spec (beware, it's large). A sample of the output looks like:

Validating spec file (openapi_kubernetes_1.8.json)
attribute definitions.io.k8s.kubernetes.pkg.api.v1.PersistentVolume.description is unexpected
attribute definitions.io.k8s.kubernetes.pkg.apis.settings.v1alpha1.PodPreset.description is unexpected
attribute definitions.io.k8s.kubernetes.pkg.apis.authentication.v1.TokenReview.description is unexpected
attribute definitions.io.k8s.kubernetes.pkg.api.v1.NodeSelectorRequirement.description is unexpected
attribute definitions.io.k8s.kubernetes.pkg.api.v1.VolumeMount.description is unexpected
attribute definitions.io.k8s.kubernetes.pkg.api.v1.StorageOSPersistentVolumeSource.description is unexpected
...
Exception in thread "main" io.swagger.codegen.cmd.ValidateException
	at io.swagger.codegen.cmd.Validate.run(Validate.java:34)
	at io.swagger.codegen.SwaggerCodegen.main(SwaggerCodegen.java:35)

The invalid definitions are found at the end of the spec file.

Command line used for generation

swagger-codegen validate -i openapi_kubernetes_1.8.json
swagger-codegen generate -i openapi_kubernetes_1.8.json -o test_generated_client_python -l java

2.2.3 installed with Homebrew on Mac OS X 10.12.6. It will generate clients for both Java and Python, I didn't check other languages.

Steps to reproduce

Download https://raw.githubusercontent.com/kubernetes/kubernetes/release-1.8/api/openapi-spec/swagger.json and run swagger-codegen with validate and generate.

Suggest a fix/enhancement

Running the validation code before generating a client would be sufficient to catch invalid specs.

Reproduce the behavior with the Kubernetes 1.8 spec and the listed validate and generate commands. Start at io.swagger.codegen.cmd.Validate.run, then inspect SwaggerCodegen.main and the generation entry point to understand where validation is handled. Done means generation no longer proceeds for a spec that validation rejects, with behavior verified against this reproduction.