OVN: add the possibility to configure a static IPv4/IPv6 address and dynamic MAC

OVN: add the possibility to configure a static IPv4/IPv6 address and dynamic MAC

Add the possibility to configure a static IPv4 and/or IPv6 address
and get MAC address dynamically allocated. This can be done using the
following commands:

$ovn-nbctl ls-add sw0
$ovn-nbctl set Logical-Switch sw0 other_config:subnet=192.168.0.0/24
$ovn-nbctl set Logical-switch sw0 other_config:ipv6_prefix=2001::0
$ovn-nbctl lsp-add sw0 lsp0 -- lsp-set-addresses lsp0 "dynamic 192.168.0.1 2001::1"

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