kubeadm: Fix omitempty in v1beta2

[rosti]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:

There are a couple of problems with regards to the omitempty in v1beta1:

• It is not applied to certain fields. This makes emitting YAML configuration files in v1beta1 config format verbose by both kubeadm and third party Go lang tools. Certain fields, that were never given an explicit value would show up in the marshalled YAML document. This can cause confusion and even misconfiguration.

• It can be used in inappropriate places. In this case it's used for fields, that need to be always serialized. The only one such field at the moment is NodeRegistrationOptions.Taints. If the Taints field is nil, then it's defaulted to a slice containing a single control plane node taint. If it's an empty slice, no taints are applied, thus, the cluster behaves differently. With that in mind, a Go program, that uses v1beta1 with omitempty on the Taints field has no way to specify an explicit empty slice of taints, as this would get lost after marshalling to YAML.

To fix these issues the following is done in this change:

• A whole bunch of additional omitemptys are placed at many fields in v1beta2.
• omitempty is removed from NodeRegistrationOptions.Taints
• A test, that verifies the ability to specify empty slice value for Taints is included.

Which issue(s) this PR fixes:

Fixes kubernetes/kubeadm#1358
Refs kubernetes/enhancements#970 kubernetes/kubeadm#1439

Special notes for your reviewer:
/cc @kubernetes/sig-cluster-lifecycle-pr-reviews
/area kubeadm
/priority important-soon
/assign @fabriziopandini
/assign @timothysc
/assign @neolit123
/cc @dlipovetsky

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rosti

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[neolit123]

thanks @rosti
the diff LGTM.

[neolit123]

thanks @rosti
the diff LGTM.

[dixudx]

/lgtm

[dixudx]

/lgtm

[rosti]

/test pull-kubernetes-integration

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