OVN: Add support for periodic router advertisements.

OVN: Add support for periodic router advertisements.

This change adds three new options to the Northbound
Logical_Router_Port's ipv6_ra_configs option:

* send_periodic: If set to "true", then OVN will send periodic router
advertisements out of this router port.
* max_interval: The maximum amount of time to wait between sending
periodic router advertisements.
* min_interval: The minimum amount of time to wait between sending
periodic router advertisements.

When send_periodic is true, then IPv6 RA configs, as well as some layer
2 and layer 3 information about the router port, are copied to the
southbound database. From there, ovn-controller can use this information
to know when to send periodic RAs and what to send in them.

Because periodic RAs originate from each ovn-controller, the new
keep-local flag is set on the packet so that ports don't receive an
overabundance of RAs.

Signed-off-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>