DNS Query Client - raise ValueError if provided Nameservers list is e…

…mpty

There is no sense to "dig" not existing DNS backends, therefor
when Query Client object is being created with an empty list for
the namesrevers attribute, ValueError will be raised.

Change-Id: I156508e404dc2d2966a189941b0b4ff13412953f

designate_tempest_plugin/services/dns/query/query_client.py

@@ -24,14 +24,20 @@ class QueryClient(object):
 
     def __init__(self, nameservers=None, query_timeout=None,
                  build_interval=None, build_timeout=None):
-        self.nameservers = nameservers or []
+        self.nameservers = self._nameservers_not_empty(
+            nameservers or CONF.dns.nameservers)
         self.query_timeout = query_timeout or CONF.dns.query_timeout
         self.build_interval = build_interval or CONF.dns.build_interval
         self.build_timeout = build_timeout or CONF.dns.build_timeout
-
         self.clients = [SingleQueryClient(ns, query_timeout=query_timeout)
                         for ns in nameservers]
 
+    def _nameservers_not_empty(self, nameservers_list):
+        if not nameservers_list:
+            raise ValueError('Nameservers list cannot be empty and it should '
+                             'contain DNS backend IPs to "dig" for')
+        return nameservers_list
+
     def query(self, zone_name, rdatatype):
         return [c.query(zone_name, rdatatype) for c in self.clients]