[BUG] mltiple stages/instances from same site in same core no longer possible

[dkd-kaehm]

Hey @baschny @dkd-kaehm, are you aware that, with this change, it is no longer possible to use the same Solr index for multiple (in our case, test) instances?
You would need a prefix for the site identifier that can be manually added for each instance.

Originally posted by @Mabahe in #3459

We need to check and restore the functionality if necessary.