Add MaxPods to KubeletConfig

[xmudrii]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds MaxPods field to the KubeletConfig used to control the maximum number of pods per node.

Example usage:

...
controlPlane:
  hosts:
    - publicAddress: ...
      kubelet:
        maxPods: 200

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
xref #2071

Does this PR introduce a user-facing change?:

Add MaxPods field to the KubeletConfig used to control the maximum number of pods per node

/assign @kron4eg

[xmudrii]

/hold
I forgot to update example config

[xmudrii]

/hold cancel

[kron4eg]

/lgtm
/approve
⚡ :this_is_fine:

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kron4eg

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Needs approval from an approver in each of these files:

• OWNERS [kron4eg]

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[kubermatic-bot]

LGTM label has been added.

Git tree hash: c000bc30a4183fc40723b73e816c94bc1e8ef733

[xmudrii]

/cherrypick release/v1.4

[kubermatic-bot]

@xmudrii: #2075 failed to apply on top of branch "release/v1.4":

Applying: Add MaxPods to KubeletConfig
Using index info to reconstruct a base tree...
M	docs/api_reference/v1beta2.en.md
M	pkg/apis/kubeone/types.go
M	pkg/apis/kubeone/v1beta2/types.go
M	pkg/templates/kubeadm/v1beta2/kubeadm.go
M	pkg/templates/kubeadm/v1beta3/kubeadm.go
Falling back to patching base and 3-way merge...
Auto-merging pkg/templates/kubeadm/v1beta3/kubeadm.go
Auto-merging pkg/templates/kubeadm/v1beta2/kubeadm.go
Auto-merging pkg/apis/kubeone/v1beta2/types.go
Auto-merging pkg/apis/kubeone/types.go
Auto-merging docs/api_reference/v1beta2.en.md
CONFLICT (content): Merge conflict in docs/api_reference/v1beta2.en.md
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Add MaxPods to KubeletConfig
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherrypick release/v1.4

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