Fix NS records in installation

Our installation added two final dots to the NS records,
so the records were invalid, Bind ignored the entire zone,
and name resolution didn't work.

Fix this error and add a check for empty DNS labels to the validator

ipalib/util.py

@@ -266,6 +266,9 @@ def validate_hostname(hostname, check_fqdn=True, allow_underscore=False):
     if hostname.endswith('.'):
         hostname = hostname[:-1]
 
+    if '..' in hostname:
+        raise ValueError(_('hostname contains empty label (consecutive dots)'))
+
     if '.' not in hostname:
         if check_fqdn:
             raise ValueError(_('not fully qualified'))