More hermetic, unified builds
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.
