Enable parallel image pulls for Kubelet

[ahmedwaleedmalik]

What this PR does / why we need it:
With these changes, the Kubelet can now pull multiple images at the same time instead of pulling one image at a time. I have added a maximum limit for concurrent pulls as well.

References:

• https://kubernetes.io/docs/reference/config-api/kubelet-config.v1beta1/#:~:text=number.%20Default%3A%20100-,serializeImagePulls,-bool
• https://kubernetes.io/docs/reference/config-api/kubelet-config.v1beta1/#:~:text=details.%20Default%3A%20true-,maxParallelImagePulls,-int32

Which issue(s) this PR fixes:

Fixes #

What type of PR is this?

/kind feature

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

Enable parallel image pulling for Kubelet with a parallel pull limit of 10

Documentation:

NONE

[xrstf]

/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: a7f89ac6c99c3dfdfb8678b5d2736274957da1b4

[kubermatic-bot]

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