Add new flag to allow CA as text argument for kubelet client

[csauoss]

What this PR does / why we need it:

• This PR adds support for new flag --kubelet-certificate-authority-text to allow passing CA as text for kubelet client. Added corresponding unit tests and updated docs.
• Also corrected the flag name in an existing test case

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1256

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This pull-request has been approved by: csauoss
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