Configurable instance domains

Currently, there is no way for a user booting an instance to tell the instance’s guest operating system what is the domain name of the instance. Using Fully Qualified Domain Names (FQDN) as the instance hostname was unintentionally allowed for a period of time, but with the merging of [1] that ability went away. This spec proposes a new API attribute to enable users to set the domain name of their instances.

[1]https://review.opendev.org/c/openstack/nova/+/764482