ovn-northd: support IPAM with externally specified MAC

ovn-northd: support IPAM with externally specified MAC

The current IPAM implementation allocates both a MAC address and
an IPv4 address when dynamic address allocation is requested. This
patch adds the ability to specify a fixed MAC address for use with
dynamic IPv4 address allocation.

Example:
   ovn-nbctl lsp-set-addresses p1 "00:01:02:03:04:05 dynamic"

Acked-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>
Signed-off-by: Lance Richardson <lrichard@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>