CHORIA_SRV_DOMAIN not getting used from environment variable

[treydock]

Now that Choria 0.28.0 is released trying to validate #2097

$ export CHORIA_SRV_DOMAIN=dev.company.net
$ choria tool config plugin.choria.srv_domain
<SNIP>
Configuration item: plugin.choria.srv_domain

║        Value: <GCP project domain, not dev.company.net>
║          URL: https://choria.io/docs/deployment/dns/
║    Data Type: string
║  Environment: CHORIA_SRV_DOMAIN
║
║ The domain to use for SRV records, defaults to the domain the
║ server FQDN is in
╙─

It looks like the lack of a default is preventing this code path from getting used. Will come up with a simple fix.