Enhance the test_multicast_between_vms_on_same_network test

This patch is enhancing the test_multicast_between_vms_on_same_network
test to fit the different scenarios that can be encountered when using
multicast:

1) When IGMP snooping is enabled and the multicast group address *is not*
   in the 224.0.0.X range [0], asserts that the multicast traffic *is not*
   flooded.

2) When IGMP snooping is not enabled, asserts that the multicast traffic
   is flooded and the unregistered VM gets it.

3) When IGMP snooping is enabled and the multicast group addres *is* in
   the 224.0.0.X range [0], asserts that the multicast traffic *is*
   flooded.

In order to make those assertions, a new VM is being launched as part of
the test running tcpdump to verify whether the traffic is reaching it or
not.

A new configuration option called "is_igmp_snooping_enabled" has been
added.

[0] https://tools.ietf.org/html/rfc4541 (See section 2.1.2)

Change-Id: I8af041925119463c7199238988f0133e8d993a8f
Signed-off-by: Lucas Alvares Gomes <lucasagomes@gmail.com>
diff --git a/neutron_tempest_plugin/config.py b/neutron_tempest_plugin/config.py
index 51fbafc..4e21879 100644
--- a/neutron_tempest_plugin/config.py
+++ b/neutron_tempest_plugin/config.py
@@ -123,6 +123,12 @@
                 default=False,
                 help='Allow creation of shared resources.'
                      'The default value is false.'),
+    cfg.BoolOpt('is_igmp_snooping_enabled',
+                default=False,
+                help='Indicates whether IGMP snooping is enabled or not. '
+                     'If True, multicast test(s) will assert that multicast '
+                     'traffic is not being flooded to all ports. Defaults '
+                     'to False.'),
 ]
 
 # TODO(amuller): Redo configuration options registration as part of the planned