ovn: specify options:nat-addresses as "router"

ovn: specify options:nat-addresses as "router"

Currently in OVN, the "nat-addresses" in the "options" column of a
logical switch port of type "router" must be specified manually.
Typically the user would specify as "nat-addresses" all of the NAT
external IP addresses and load balancer IP addresses that have
already been specified separately on the router.

This patch allows the logical switch port's "nat-addresses" to be
specified as the string "router".  When ovn-northd sees this string,
it automatically copies the following into the southbound
Port_Binding's "nat-addresses" in the "options" column:
    The options:router-port's MAC address.
    Each NAT external IP address (of any NAT type) specified on the
    logical router of options:router-port.
    Each load balancer IP address specified on the logical router of
    options:router-port.
This will cause the controller where the gateway router resides to
issue gratuitous ARPs for each NAT external IP address and for each
load balancer IP address specified on the gateway router.

Signed-off-by: Mickey Spiegel <mickeys.dev@gmail.com>
Acked-by: Gurucharan Shetty <guru@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>