Backport spec.users.passwd should be optional/omitempty

[vincepri]

Relevant v1alpha3 commit: kubernetes-sigs/cluster-api@4c5edd4

/kind bug
/priority critical-urgent
/help

[k8s-ci-robot]

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In response to this:

Relevant v1alpha3 commit: kubernetes-sigs/cluster-api@4c5edd4

/kind bug
/priority critical-urgent
/help

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[vincepri]

We should also make sure that other fields marked optional have the omitempty tag properly set

[nader-ziada]

/assign