Change from "extensions.v1beta1" to "networking.v1beta1"

[bowei]

Change API group from "extensions" to "networking"

This migrates our code to use the networking.v1beta1.Ingress* API
types instead of extensions.v1beta1. This also fixes fixture YAMLs
to use "networking.k8s.io" as the fully-qualified resource kind.

[bowei]

/assign @rramkumar1 @freehan

[bowei]

cc: @liggitt

[rramkumar1]

@bowei We would need to update the YAML's in our GCP documentation as well.

[1] https://cloud.google.com/kubernetes-engine/docs/concepts/ingress

[freehan]

Will it still recognize ingress objects in extension api group?

I assume API machinery should have handled this kind of backward compatibility, right?

[bowei]

Yes, asked Jordan, this should work as long as it is used with v1.15+ k8s clusters

[freehan]

/lgtm
/approve

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