Add doc for using custom CA

[joshbranham]

This adds a doc on how to create a kops cluster using a custom certificate authority, or with a previously kops generated CA keypair. I saw some PR issues on allowing this but no clear documentation on how to go about implementing it. This is how I got it to work, more than happy to adjust if anything is not totally accurate.

[k8s-ci-robot]

Hi @joshbranham. Thanks for your PR.

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[mikesplain]

/ok-to-test

[mikesplain]

This looks great! Leaving open in case anyone else with experience on this matter has comments.

/lgtm

[joshbranham]

/test pull-kops-e2e-kubernetes-aws

[mikesplain]

Thanks @joshbranham!

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: joshbranham, mikesplain

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