OVN: update RA next_announce according to {min, max}_interval

OVN: update RA next_announce according to {min, max}_interval

Update RA next_announce whenever min_interval and/or max_interval are
updated in sbrec_port_binding option. In the current implementation
if ipv6_ra_configs:send_periodic is set to true before setting
ipv6_ra_configs:{min,max}_interval, next_announce will be set using
default values and it will not be updated until we send the first IPv6
router advertisement

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>