More hermetic, unified builds

[adrianludwin]

This commit makes two significant changes:

• Includes the kustomize tool so that we don't need to either fetch it
  from the internet or rely on one being installed locally.
• Replaces portions of the Cloud Build process with the equivalent steps
  from the Makefile so they can't get out of sync.

As a side effect, this required a bit of refactoring in the makefile,
and I've moved some lines around to be more closely located with the
relevant recipes.

I tried removing kubebuilder (I thought we just needed controller-gen)
but it contains elements required by Ginkgo. Since it's rather large
(>50MB) and doesn't affect the correctness of the binaries as they're
built (only tested), I decided not to check it in. I did upgrade the
version we use from a 2.0 alpha to the latest release (as of Aug 2020).

Finally, this includes a small docs change that was mistakenly left out
of an earlier PR.

Tested: made all targets locally; confirmed kubectl hns version gave
the expected result; pushed a test tag to my repo and used the
HNC_RELEASE_REPO_OWNER flag to build a fake release from that tag.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adrianludwin

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[adrianludwin]

/assign @rjbez17
/assign @yiqigao217
/cc @GinnyJI

[yiqigao217]

Thanks for the explanations! @adrianludwin can you help me resolve the comments (somehow I don't have that option).

/lgtm
/hold
/assign @rjbez17

[adrianludwin]

/wip

This conflicts with #1032 so I need to revise

[k8s-ci-robot]

@adrianludwin: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-mtb-test	be67541	link	/test pull-mtb-test

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[adrianludwin]

/hold cancel

Updated to include multiplatform kubectl plugins. Tested by releasing to https://github.com/adrianludwin/multi-tenancy/releases/tag/hnc-v0.6.0-test1.

[adrianludwin]

@yiqigao217 Can you please re-review? Note that the hold is gone.
@rjbez17 Are you interested in reviewing this?

Thanks, A

[yiqigao217]

lgtm just 1 nit

[yiqigao217]

/lgtm