Automated cherry pick of #44570 #44862

[liggitt]

Cherry pick of #44570 #44862 on release-1.6.

#44570: Explicit namespace from kubeconfig should override in-cluster
#44862: Stop treating in-cluster-config namespace as an override

* kubectl commands run inside a pod using a kubeconfig file now use the namespace specified in the kubeconfig file, instead of using the pod namespace. If no kubeconfig file is used, or the kubeconfig does not specify a namespace, the pod namespace is still used as a fallback.
* Restored the ability of kubectl running inside a pod to consume resource files specifying a different namespace than the one the pod is running in.

[k8s-reviewable]

This change is 

[enisoc]

LGTM once all parent PRs merge.

[liggitt]

parents are merged. @smarterclayton for lgtm

[smarterclayton]

/lgtm

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt, smarterclayton

Needs approval from an approver in each of these OWNERS Files:

• staging/src/k8s.io/client-go/OWNERS [smarterclayton]
• test/OWNERS [liggitt,smarterclayton]

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[liggitt]

tagging per #44877 (comment)

[k8s-github-robot]

Automatic merge from submit-queue

[jimmycuadra]

Was this included in 1.6.2? If not, do we have an ETA on a 1.6.3 release? Our custom deployment automation is completely broken by this. We had been continuing to use kubectl 1.4.x as a workaround (see #43662) while our cluster was running 1.5.x, but after upgrading to 1.6.2, kubectl 1.4.x no longer works and we were forced to upgrade it. kubectl 1.4 produces this error when used to apply resources to a 1.6 cluster:

error: group map[componentconfig:0xc8203ad420 rbac.authorization.k8s.io:0xc8204d0000 certificates.k8s.io:0xc8203ad3b0 :0xc8203acee0 apps:0xc8203acf50 authentication.k8s.io:0xc8203acfc0 authorization.k8s.io:0xc8203ad110 autoscaling:0xc8203ad180 batch:0xc8203ad340 extensions:0xc8203ad500 federation:0xc8203aca10 storage.k8s.io:0xc82010a000 policy:0xc8203ad570] is already registered

[enisoc]

@jimmycuadra This will be included in v1.6.3, which I plan to cut sometime this week.