feat(config): read both owner and domain from domain

- retro-compatible change: keep using host/owner if set
- otherwise extract owner and eTLD+1 from domain field
- documentation updated to only use the `domain` field

docs/inwx.md

@@ -10,7 +10,6 @@
     {
       "provider": "inwx",
       "domain": "domain.com",
-      "owner": "@",
       "username": "username",
       "password": "password",
       "ip_version": "ipv4",
@@ -22,8 +21,7 @@
 
 ### Compulsory parameters
 
-- `"domain"`
-- `"owner"` is the resource record owner and can be `"@"` for the root domain or for example `"sub"` for the subdomain `sub.domain.com`
+- `"domain"` is the domain to update. It can be `example.com` (root domain) or `sub.example.com` (subdomain of `example.com`).
 - `"username"`
 - `"password"`