Deprecated the domain-level option in ipa-server-install

https://fedorahosted.org/freeipa/ticket/5907

Reviewed-By: Martin Basti <mbasti@redhat.com>

ipaserver/install/server/install.py

@@ -1368,6 +1368,7 @@ class Server(BaseServer):
         int, constants.MAX_DOMAIN_LEVEL,
         description="IPA domain level",
         cli_name='domain-level',
+        deprecated=True,
     )
 
     @domainlevel.validator