Using boot disks to provision hosts

[Lennonka]

https://theforeman-foreman-documentation-preview-pr-3922.surge.sh/nightly/Provisioning_Hosts/index-foreman-el.html#using-boot-disks-to-provision-hosts

In upstream we don't ship the bootdisk plugin by default and this procedure misses instructions to enable it. In Satellite it's enabled by default so there it isn't an issue.

Originally posted by @ekohl in #3922 (review)