New feature?: Allow automatic downloads of Kubernetes OpenAPI specs

[fllaca]

Kubernetes OpenAPI specs are publicly available in Github at https://raw.githubusercontent.com/kubernetes/kubernetes/v1.17.0/api/openapi-spec/swagger.json. We could add some local filesystem cache of K8s swagger specs that scheriff can populate on demand when running validations without the -s path/to/swagger.json option. The example usage would be:

specifying K8s version:

# this will download https://raw.githubusercontent.com/kubernetes/kubernetes/v1.17.0/api/openapi-spec/swagger.json and check path/to/k8s/config against it

scheriff --k8s-version=1.17.0 -f path/to/k8s/config

assuming latest stable k8s version:

# this would detect latest stable kubernetes version and download it behind the scenes:

scheriff -f path/to/k8s/config