Allow setting of independent custom domain for incus inventory (#11555)
Allowing the domain suffix to be appended independent of the `host_fqdn`
setting enables the inventory plugin to construct proper FQDNs if a
network has the `dns.domain` property set. Otherwise you would always
end up with something like `host01.project.local.example.net` despite
`host01.example.net` being the expected result.
(cherry picked from commit 71f8c15d2e)
Co-authored-by: Roland Sommer <rol@ndsommer.de>