Add Neutron distributed virtual router

DVR (distributed virtual router) related column and menu
are displayed on when DVR is enabled. To enable DVR support,
you need to set enable_distribute_router in local_settings to True
and run Neutron with DVR support.

neutron_policy.json is updated to allow policy check for DVR attribute.

Implements blueprint enhance-horizon-for-dvr
Change-Id: I422508395d35a6160c671df616aa6a6e90e5de36