Comment 3 for bug 1903908

it is used for the config drive that all its needed for.
specifically so we can set the hostname in the configdrive by appending the dhcp domain
https://github.com/openstack/nova/blob/7f4afc1a0fbc721731a0c88bb91cada8f4e4ed11/nova/api/metadata/base.py#L529-L535

if we dont set it on the compute hosts then it will set the host name without the domain.