Collect namespaces of deployed resources.

[tejal29]

In #2495, @corneliusweig noticed for a skaffold run without --namespace flag, we make a call for deployment health-check and port-forwarding

client.AppsV1().Deployments(ns).List(metav1.ListOptions{
		LabelSelector: l.K8sManagedByLabelKeyValueString(),
	})

and ns is "".
Similarly, during port-forwarding, we do the same

services, err := clientset.CoreV1().Services("").List(metav1.ListOptions{
		LabelSelector: label,
	})

In context.RunContext, we already gather namespaces set in the kubecontext as well as on command line.
This PR is collect all the namespaces from kubectl manifests and helm releases if they are set individually in the manifests.

This PR is not enough for the bug fix for #2495. We need to followup with a PR where if runcontext.Namespaces is empty, we need to use "default" namespace.

[nkubala]

I think the idiomatic way to write this would be

for i, ns := range replacer.namespaces {
  namespaces[i] = ns
}

[tejal29]

this is map so its a key value pair.
I will use append. I first thought it allocates a new memory every time and will be inefficient.
But turns out, it only allocates a new memory if the size is less than what was allocated.

namespaces := make([]string, 0, len(replacer.namespaces))

for  ns := range replacer.namespaces {
   namespaces = append(namespaces, ns)
}

[codecov]

Codecov Report

Merging #2640 into master will increase coverage by 0.07%.
The diff coverage is 61.26%.

Impacted Files	Coverage Δ
pkg/skaffold/runner/notification.go	0% <0%> (ø)	⬆️
pkg/skaffold/runner/timings.go	14% <0%> (ø)	⬆️
pkg/skaffold/event/event.go	88.59% <0%> (-1.21%)	⬇️
pkg/skaffold/deploy/deploy.go	100% <100%> (ø)
pkg/skaffold/runner/deploy.go	72.22% <100%> (+1.63%)	⬆️
pkg/skaffold/runner/runcontext/context.go	72.09% <100%> (+9.59%)	⬆️
pkg/skaffold/deploy/kustomize.go	72.32% <33.33%> (-1.76%)	⬇️
pkg/skaffold/deploy/kubectl.go	65.51% <35.71%> (-1.45%)	⬇️
pkg/skaffold/deploy/helm.go	66.54% <50%> (-1%)	⬇️
pkg/skaffold/deploy/util.go	78.94% <72.22%> (+10.94%)	⬆️
... and 5 more

[priyawadhwa]

LGTM, just a couple small comments

[priyawadhwa]

nit: I feel like I typically see "OK bools" returned as the second argument--

for example, with maps:

val, ok := myMap[key]