In the ML2/OVS when igmp_snooping is enabled but there is no
external querier multicast traffic will stop working after few minutes
as packets will not be flooded to tunnel/external bridges.
So this patch sets "mcast-snooping-disable-flood-unregistered" option
of the br-int to False (default value) even when igmp_snooping is
enabled in the neutron-ovs-agent's config file.
Additionally it configures "mcast-snooping-flood-reports" and
"mcast-snooping-flood" on patch ports in br-int to True.
That way we can provide best effort snooping: multicast isolation where
IGMP queriers are available and flood everywhere else?
Closes-Bug: #1884723
Change-Id: Iefa0044dba9e92592295a79448e5d57d9e14a40b