Add ready-pods-only annotation

[alfredkrohmer]

This annotation can be used to only register IPs of pods that are in
"Ready" state (instead of the default behavior of registering all
running pods) in DNS.

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[alfredkrohmer]

This closes #878.

[alfredkrohmer]

/assign @linki

[njuettner]

I don't know if this makes sense to check the annotation from a service for each pod again and again.
I think this needs to be solved in a better way, WDYT?

[alfredkrohmer]

Good point, I will pull it out of the loop.

[Raffo]

@devkid can you please resolve the conflict?

@njuettner Apart from the conflicts, the changes look good to me, are there any other reasons why we were waiting to move forward with this PR?

[alfredkrohmer]

@Raffo See @nmiculinic's comment in #878. I want to implement this in a different way (which actually makes more sense). I'll follow up today.

[alfredkrohmer]

@Raffo superseded by #1005