Fix AttributeError on testtools missing MismatchError

It sounds like MismatchError is borderline between part
of a public interface of testtools, and may or may not be
available depending of testtoools version.

Get rid of the nonsense altogether. The helper method was
wrong in that it shouldn't have been asserting a result if an
expected condition was not that. This commit removes the
broken helper method in favor of directly calling the 
get_dns_servers method where necessary.

Closes-bug: #1424482

Change-Id: Iaa611f833f70a528f38fe8cc9d9add0047bf51e1
diff --git a/tempest/scenario/test_network_basic_ops.py b/tempest/scenario/test_network_basic_ops.py
index a7d28bb..be6f8fd 100644
--- a/tempest/scenario/test_network_basic_ops.py
+++ b/tempest/scenario/test_network_basic_ops.py
@@ -17,7 +17,6 @@
 import re
 
 import testtools
-from testtools.tests import matchers
 
 from tempest.common.utils import data_utils
 from tempest import config
@@ -432,16 +431,6 @@
             should_connect=True, msg="after updating "
             "admin_state_up of router to True")
 
-    def _check_dns_server(self, ssh_client, dns_servers):
-        servers = ssh_client.get_dns_servers()
-        self.assertEqual(set(dns_servers), set(servers),
-                         'Looking for servers: {trgt_serv}. '
-                         'Retrieved DNS nameservers: {act_serv} '
-                         'From host: {host}.'
-                         .format(host=ssh_client.ssh_client.host,
-                                 act_serv=servers,
-                                 trgt_serv=dns_servers))
-
     @testtools.skipUnless(CONF.scenario.dhcp_client,
                           "DHCP client is not available.")
     @test.attr(type='smoke')
@@ -486,7 +475,15 @@
         private_key = self._get_server_key(server)
         ssh_client = self._ssh_to_server(ip_address, private_key)
 
-        self._check_dns_server(ssh_client, [initial_dns_server])
+        dns_servers = [initial_dns_server]
+        servers = ssh_client.get_dns_servers()
+        self.assertEqual(set(dns_servers), set(servers),
+                         'Looking for servers: {trgt_serv}. '
+                         'Retrieved DNS nameservers: {act_serv} '
+                         'From host: {host}.'
+                         .format(host=ssh_client.ssh_client.host,
+                                 act_serv=servers,
+                                 trgt_serv=dns_servers))
 
         self.subnet.update(dns_nameservers=[alt_dns_server])
         # asserts that Neutron DB has updated the nameservers
@@ -501,9 +498,7 @@
             subnet-update API call returns.
             """
             ssh_client.renew_lease(fixed_ip=floating_ip['fixed_ip_address'])
-            try:
-                self._check_dns_server(ssh_client, [alt_dns_server])
-            except matchers.MismatchError:
+            if ssh_client.get_dns_servers() != [alt_dns_server]:
                 LOG.debug("Failed to update DNS nameservers")
                 return False
             return True