Fix creation of the administrator_keys_file file with many users

[mauriciopoppe]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Fixes a couple of things:

• It's possible that there are multiple valid users in sshKeys e.g. (prow, root, ...), while all of them need to be added to the administators_authorized_keys it looks like for each user this file is cleaned first and then the user keys are added overwriting the keys written for a previous user!
• I saw that the user_keys_file and the administrator_keys_file are recreated on every iteration which leads to race conditions, we could instead treat these files as append only and only add new keys that weren't seen in the file, with this the file no longer needs to be removed, the tradeoff is that revoked keys can't be removed but may not happen in practice

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

/cc @pjh

[mauriciopoppe]

/sig windows

[pjh]

/triage accepted
/priority important-soon

[pjh]

I gave this a cursory review and I sort-of understand what it does. This is delicate legacy code that's specifically testonly, hopefully in 2022 Windows VMs on GCE will gain ssh capability by default and we'll be able to remove this.

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mauriciopoppe, pjh

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