kubeadm: fix a bug in defaulting apiserver "authorizaion-mode"

[neolit123]

What type of PR is this?

/kind bug

What this PR does / why we need it:

When the flag is defaulted before writing the apiserver manifest, the input "cfg" object should not be mutated.

If the "cfg" is mutated, the upload ClusterConfiguration to the cluster will include the defaulting, which is not needed.

Which issue(s) this PR fixes:

Fixes kubernetes/kubeadm#3029

Special notes for your reviewer:

Does this PR introduce a user-facing change?

kubeadm: avoid uploading a defaulted flag value "--authorization-mode=Node,RBAC" for the kube-apiserver in the ClusterConfiguration stored in the "kube-system/kubeadm-config" ConfigMap. "Node,RBAC" are already the kubeadm defaults for this flag, so this action is redundant.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: neolit123

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[neolit123]

/triage accepted
/priority backlog

/cc @SataQiu @pacoxu
just a minor bug fix

[pacoxu]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 82eaa1b74bd06213c2e1660095e35e6d58024e06

[carlory]

LGTM

[pacoxu]

/retest
unrelated failure

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/retest