Added Windows Custom Image. Only works in dcos, but can be added to k8s

[yakman2020]

What this PR does / why we need it:
Allows the use of custom windows system disk images in the VLabs API deployments. This in turn allows us to use confidential daily build, or specially provisioned images. This is implemented completely for DC/OS. Someone in the k8s team will probably want to apply the changes the k8s parts.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

While this is implemented completely for DC/OS, it modifies the WindowsProfile so should be usable in any of the different code legs, such as k8s or docker swarm with fairly simple mods.

Release note:

[anhowe]

reference the assigned value. Looks at examples from Subnet or VMSize

[yakman2020]

done.

[anhowe]

How is this different from WindowsImageSourceUrl?

[yakman2020]

For now, it isn't except its in variables() rather than parameters().

[anhowe]

I would recommend apply to whole cluster (all agent pools). or per agent pool, but not both. Which matches your scenario better?

[yakman2020]

Whole cluster. I beleive that is doing that. Am I missing something?

[anhowe]

If "whole cluster" is your scenario, I think you can completely remove the per node pool change you made in parts/agentparams.t.

[anhowe]

I've recommended some changes. Since this is only DCOS, please add validation to fail if this options was specified for Kubernetes.

[jackfrancis]

@yakman2020 this looks good to go except for the cruft in pkg/acsengine/json.go.

[jackfrancis]

Also, to address @anhowe's comment, please add something like this to pkg/api/vlabs/validate.go under the func (a *Properties) Validate(isUpdate bool) error { function:

if a.OrchestratorProfile.OrchestratorType != DCOS {
	if a.WindowsProfile.HasCustomImage() {
		return fmt.Errorf("Windows custom image is only supported for DC/OS orchestrator type")
	}
}

[yakman2020]

validation added.

[yakman2020]

@anhowe Added validation, removed per host custom image name.

[jackfrancis]

lgtm