Create HierarchicalNamespace CRD

[yiqigao217]

This CRD is created as the UX of the HNC self-service namespace. See
design at http://bit.ly/hnc-self-serve-ux.

Tested on a GKE cluster by "kubectl apply -f" the generated CRD yaml
file. The CRD was created with success. I also added a sample hns.yaml
and ran "kubectl create -f", the hns object was created and showed up
when I ran "kubectl get hns".

The possibility of "kubectl create hns" is still under investigation,
since the "kubectl create" only supports a selected list of built-in
resource types (even "kubectl create pod" is not supported). We will see
if "kubectl create" a CR object is possible.

Part of #457

[adrianludwin]

lgtm after one last change

[adrianludwin]

lgtm. Can you confirm that if you just say kubectl create hns bar -n foo (or kubectl create hierarchicalnamespace bar -n foo, then an empty HNS object called "bar" gets created in namespace "foo"?

[adrianludwin]

/assign @rjbez17

[adrianludwin]

/hold
/lgtm
/approve

For Ryan to review.

[yiqigao217]

I added a sample hns.yaml for kubectl to apply to create an hns object and updated the commit body with the explanation that all commands kubectl apply -f crd.yaml, kubectl create -f hns.yaml and kubectl get hns worked while kubectl create hns not.

It's a kubectl related issue that CR objects can be created by kubectl create -f but not directly by kubectl create, which is out of the scope of this PR.

[yiqigao217]

/assign @sophieliu15

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[adrianludwin]

Ok, pity that we can't do k create hns, we'll just have to go with k hns create for this one case.

/shrug

I'm ok with this one @rjbez17 is. I think he's OOO today so perhaps he'll reply tomorrow, if not lmk and I'll approve this myself.

[rjbez17]

/lgtm
/approve
/hold cancel

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adrianludwin, rjbez17, yiqigao217

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