Preserve existing namespace when using kubectl set --local

[liggitt]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Uses existing namespace info in manifests to determine whether to preserve namespace info when running commands locally without RESTMapper info

Which issue(s) this PR fixes:
Fixes kubernetes/kubectl#639

Does this PR introduce a user-facing change?:

Preserves existing namespace information in manifests when running `kubectl set ... --local` commands

/sig cli
/cc @seans3

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt

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Needs approval from an approver in each of these files:

• pkg/kubectl/OWNERS [liggitt]
• staging/src/k8s.io/cli-runtime/OWNERS [liggitt]
• test/fixtures/pkg/kubectl/OWNERS [liggitt]

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[seans3]

/retest

[seans3]

/lgtm