Infer the registry FQDN from the server FQDN

The registry FQDN should have the same domain and framework as
the server FQDN, as such, no option to set a different value in the CLI args

usr/sbin/registercloudguest

@@ -214,12 +214,6 @@ argparse.add_argument(
     dest='user_smt_fqdn',
     help=help_msg
 )
-help_msg='The target registry server FQDN. '
-argparse.add_argument(
-    '--smt-registry-fqdn',
-    dest='user_registry_fqdn',
-    help=help_msg
-)
 help_msg='The target update server IP. '
 help_msg+='Use in exceptional cases only'
 argparse.add_argument(
@@ -362,7 +356,8 @@ if args.user_smt_ip:
     elif isinstance(smt_ip, ipaddress.IPv4Address):
         smt_xml += 'SMTserverIP="%s" ' % args.user_smt_ip
     smt_xml += 'SMTserverName="%s" ' % args.user_smt_fqdn
-    smt_xml += 'SMTregistryName="%s"' % args.user_registry_fqdn
+    registry_fqdn = 'registry-{}'.format(args.user_smt_fqdn.split('-')[2])
+    smt_xml += 'SMTregistryName="%s"' % registry_fqdn
     smt_xml += '/></regionSMTdata>'
     region_smt_data = etree.fromstring(smt_xml)
 else: