Add Conflict exception in test_floating_ips_actions case

We modify the error return code in nova api both v2 and v2.1, so we need to
reflect this to tempest test cases test_associate_already_associated_floating_ip
changes : add HTTPConflict

Nove review link: https://review.openstack.org/#/c/127889/

Change-Id: Icbb7b0aa46c864993290749e9a6711f3fa224255
diff --git a/tempest/api/compute/floating_ips/test_floating_ips_actions.py b/tempest/api/compute/floating_ips/test_floating_ips_actions.py
index ba66ab9..3bb7d19 100644
--- a/tempest/api/compute/floating_ips/test_floating_ips_actions.py
+++ b/tempest/api/compute/floating_ips/test_floating_ips_actions.py
@@ -130,7 +130,8 @@
 
         # Make sure no longer associated with old server
         self.assertRaises((exceptions.NotFound,
-                           exceptions.UnprocessableEntity),
+                           exceptions.UnprocessableEntity,
+                           exceptions.Conflict),
                           self.client.disassociate_floating_ip_from_server,
                           self.floating_ip, self.server_id)