bgpd: An ability to set attributes for default-originate via route-map

bgpd: An ability to set attributes for default-originate via route-map

With this change, we are able to set attributes via route-map to the default
route. It's useful in cases where we have two or more spines and we want to
prefer one router over others for leaves. This simplifies configuration instead
of using 'network 0.0.0.0/0' or 'ip route 0.0.0.0/0 ...' and 'redistribute
static' combination.

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>