Increase default IPv6 router advertisement interval

The current values of min:3 and max:10 mean radvd is sending
an RA about every 7 seconds, which can be excessive when we
have thousands of routers.  Let's relax it by 10x since most
VMs will send a Router Solicition at boot, obviating the need
for a small interval.

Related-Bug: #1532338
Change-Id: Ie0a411f67d10ec1469841d70fb643409f77be56f

neutron/agent/linux/ra.py

@@ -40,10 +40,10 @@ OPTS = [
                default='$state_path/ra',
                help=_('Location to store IPv6 RA config files')),
     cfg.IntOpt('min_rtr_adv_interval',
-               default=3,
+               default=30,
                help=_('MinRtrAdvInterval setting for radvd.conf')),
     cfg.IntOpt('max_rtr_adv_interval',
-               default=10,
+               default=100,
                help=_('MaxRtrAdvInterval setting for radvd.conf')),
 ]