Ignore ambassador-hosts with invalid annotations

[olemarkus]

Description

If e.g the ambassador service annotation points to an nonexisting service, external-dns will not update any DNS records. This PR makes external-dns ignore the host and carry on instead.

Didn't provide any unit tests as this code didn't seem to have anything like a fake client that knows about ambassador hosts. Did however validate the logic in a live cluster.

[szuecs]

Thanks code looks fine.
@olemarkus what about tests?

[olemarkus]

As mentioned, its not entirely straightforward to introduce unit test to that code. Would need to wire up the dynamic client and such. It would be a significantly larger PR alone. But this source is important to us, so I don't mind doing another PR for that. It will take a few days maybe.

[olemarkus]

@szuecs added test only for this particular bug now.

[szuecs]

Thanks @olemarkus !

[szuecs]

/approve
/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

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[KyleMartin901]

Hi @szuecs and @olemarkus I have added tests in my PR that also updates the Ambassador source to use label and annotation filters #2633. If you wouldn't mind taking a look that would be awesome. I will have to refactor based on this PR

[olemarkus]

@KyleMartin901 The PR looks good, I think. Ping me when you've refactored. I can review, but I cannot merge, unfortunately.

[KyleMartin901]

Thanks, @olemarkus I managed to find some time to rebase the tests today so 🤞 it's all good.