Werk #13864: Host registration: Enable on sites with disabled Setup & check site name

The host registration via the cmk-agent-ctl command failed for sites with disabled Setup.

Furthermore, upon registration, we now check if the site where the host is registering is also the site on which it is monitored (only relevant in distributed setups). If not, the registration fails.

Finally, upon registration we also ensure that the host is not a cluster host, since cluster hosts have no direct physical or virtual counterpart which needs to be registered.

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