Multiple subnets or domains

[Lennonka]

https://theforeman-foreman-documentation-preview-pr-3922.surge.sh/nightly/Provisioning_Hosts/index-foreman-el.html#Multiple_Subnets_or_Domains_Using_Installer_provisioning

Needs to move to the new Integrating provisioning services guide.

Originally posted by @ekohl in #3922 (review)