Don't try building TLS for etcd-manager if not using etcd-manager

[johngmyers]

Fixes #9298
/kind bug

[moshevayner]

/lgtm

[rifelpet]

/approve

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[justinsb]

Ah - the distinction I tend to make is that Get returns an error when the thing is not found, Find returns nil when the thing is not found. If we've changed that semantic for FindCert that might well cause us some trouble... I'll look at renaming FindCert -> GetCert and see which way we need to go...

Edit: Actually looks fine, FindCert still returns (nil, nil) when not found; it's just the additional cert afterwards. On returning error vs not, being stricter here is cleaner, the risk is that it fails nodeup when there's nothing that can be done. I'm not very consistent on what we should do here, but I think in retrospect failing fast rather than failing subtly is better so 👍