Werk #12147: Baked agents implicitly lose their signature when new hosts are added

This Werk fixes a misbehavior that was caused by the combination of the two features from Werks #11403 and #11248.

When a host gets linked to an already existing agent package, the matching signature will get invalidated. The reason for this is the package versioning, see Werk #11403. Additionally, agent packages will get baked automatically on adding new hosts, see Werk #11248. Hence, if an agent package that would match to a new host already exists at the time the host is added, it's signature will get invalidated without further notice, possibly leading to several errors on the next (automatic) agent updater call.

With this werk, the versioning mechanism from Werk #11248 is prevented, if the agent bakery is called only with a subset of all hosts, because this is what is done when adding a new host.

This can safely be done, because in most situations, the above mentioned versioning mechanism won't be needed for a newly added host.

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