Catching new exception while disassociating a disassociated floating ip

Because the response of disassociating floating ip api will be
changed(bug #1129719), so the tempest codes need to be updated synchronically.

Change-Id: Ie25ed338dda01cf5e52982abae9ea85386a293a9
diff --git a/tempest/tests/compute/floating_ips/test_floating_ips_actions.py b/tempest/tests/compute/floating_ips/test_floating_ips_actions.py
index ffff1f7..0f63016 100644
--- a/tempest/tests/compute/floating_ips/test_floating_ips_actions.py
+++ b/tempest/tests/compute/floating_ips/test_floating_ips_actions.py
@@ -176,7 +176,7 @@
             self.client.disassociate_floating_ip_from_server(
                 self.floating_ip,
                 self.server_id)
-        except exceptions.NotFound:
+        except (exceptions.NotFound, exceptions.BadRequest):
             pass
         else:
             self.fail('The floating IP should be associated to the second '