Document how etcd's TLS assets are managed

[markmc]

I found all of this quite bewildering to keep straight in my head, so I figured I'd try to write it down.

[hexfusion]

@markmc this is an amazing amount of work. Thank you, will review soon.

[cgwalters]

I happened to come across this when I was trying to figure out how the etcd static pod manifest is installed. Agreed, awesome work! I personally would merge this as is and do other changes as followups to avoid losing it.

It seems though the second half in particular is more about the architecture, and isn't specific to certs; maybe move that into a separate design.md?

[hexfusion]

/lgtm
/approve

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hexfusion, markmc

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[hexfusion]

yeah let's merge marks work and fine-tune from here thanks again.

[hexfusion]

/retest

[hexfusion]

/skip

[hexfusion]

/skip

[openshift-ci-robot]

@markmc: The following tests failed, say /retest to rerun all failed tests:

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ci/prow/e2e-aws-disruptive	3afb9ed	link	/test e2e-aws-disruptive

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[hexfusion]

/skip