feat: add flag to skip TLS verification

[steled]

What this PR does / why we need it:
Add possibility to skip TLS verification for self-signed certificates.

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:

- parameter `insecure` added for skipping TLS verification

Documentation:

existing doc: https://docs.kubermatic.com/kubeone/v1.9/examples/addons-backup/
PR: https://github.com/kubermatic/docs/pull/1807

[kubermatic-bot]

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[steled]

I also updated the documentation at: kubermatic/docs#1807

[kron4eg]

/lgtm
/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 3a4801f9b1c8d871e6e363ed7bdef6b38ff30f9e

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kron4eg

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[kron4eg]

/hold

waiting #3523 to merge

[kron4eg]

/retest

[kron4eg]

/hold cancel