HostDomain is like HostAddress with the support of
_ character - RFC1033
openstack services are failing to start when a hostname with underscore
_ is provided.
Example:
```
overcloud-novacompute_edge1-0.internalapi.localdomain
overcloud-novacompute_edge1-0.internalapi
```
Nova use `HostAddressOpt` to define `live_migration_inbound_addr`,
and if a hostname with underscore is present in the config file
then the service fail to start.
Example:
```
/etc/nova/nova.conf
live_migration_inbound_addr =
overcloud-novacompute_edge1-0.internalapi.localdomain
```
FQDN is a domain name that specifies its exact
location in the tree hierarchy of the Domain Name System (DNS).
Underscore are allowed by RFC1033 [1][2][3]. Indeed, while a hostname may not
contain other characters, such as the underscore character (_), other
DNS names may contain the underscore.[1][2].
Systems such as DomainKeys and service records use the underscore.
These changes allow us to use underscore with the `HostDomain`.
[1] https://www.ietf.org/rfc/rfc1912.txt
[2] https://www.ietf.org/rfc/rfc1033.txt
[3] http://domainkeys.sourceforge.net/underscore.html
Co-authored-by: Daniel Bengtsson <dbengt@redhat.com>
Change-Id: I0a0670207f96a987996d329e5efa9a5eb2ce000c
Closes-Bug: #1892044
6480356928