Add flag --enable-helm

[monopole]

This PR adds two command line flags to kustomize build

--enable-helm
--helm-command /path/to/helm

and changes behavior accordingly.

Please read examples/chart.md in this PR.

That file has been activated as an end-to-end test.
The helm plugin tests have also been activated.

Flags

Kustomize won't spawn a helm subprocess unless --enable-helm is
specified.

The helm command defaults to simply helm, and it must be on the
user's PATH. To override this requires use of the --helm-command flag.
This is a command line parameter rather than a config parameter to
mitigate config download risks.

Fields

The existing kustomization field HelmChartInflationGenerator is still
mostly honored, but now deprecated.

It's replaced by two new fields, explained in detail in this PR's helmchartargs.go file.
The fields have been split into two groups, HelmCharts and
HelmGlobals, with the former containing chart specific info, and the
latter containing general helm options.

The HelmBin, ExtraArgs from the HelmChartInflationGenerator field
are ignored because they represent a security risk which bears further examination.
The purpose served by HelmBin is now served by the --helm-command flag.

Also, the ReleaseNamespace field is ignored, as it doesn't work in
helm's template mode, and kustomize has it's own way of specifying
namespace.

ALLOW_MODULE_SPAN

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: monopole

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[monopole]

/test kustomize-presubmit-master

[monopole]

/retest

[Shell32-Natsu]

/lgtm