Create CRD validation specs in CRD manifests and as Go code

[estroz]

Description of the change: operator-sdk generate openapi generates OpenAPI validation specs for CRD manifests and as Go code under pkg/apis/<group>/<version>/zz_generated.openapi.go. operator-sdk add api also calls the openapi generator. The CRD scaffold now implements CustomRenderer to generate a CRD using controller-tools' CRD generator by default; if some CRD manifest exists, CustomRender will only set the validation spec of that CRD.

Motivation for the change: validation specs should be present for all CRD's, yaml or otherwise. Users might want to use the Go specs for Go-defined CRD's.

Note: we can remove Go spec generator, as there haven't been requests for one. I thought it would be a nice addition but not necessary. I'm also open to not having a separate generate command, and having all generation under generate k8s.

I'll add documentation in another PR since this one is already large.

Closes #256

[solcates]

Any update on this PR? Seems pretty solid, have been testing it with some basic types to CRD generation, and so far so good

[hasbro17]

LGTM after nits.

@estroz As we discussed offline I'm fine with having users using the +kubebuilder:validation tags to add the custom validation directives.

But as a follow up to this PR we should try to look into not stomping out any validation directives that are added directly to the CRD yaml manifest on running generate openapi.

[hasbro17]

Also the CHANGELOG and cli-reference guide need to be updated but it's fine if you have that lined up in follow up PR to add the docs.

[estroz]

The additions in this PR to the generated Gopkg.toml are breaking dep. I am in the process of figuring out which packages are in conflict.