bgpd: fix colored nexthops resolution

bgpd: fix colored nexthops resolution

When the SR-TE service is off, colored BGP routes are not
selected if it is recursively resolved over routes that are
colored only.

Actually, a BGP nexthop context includes the color attribute;
when an update from ZEBRA is received, there is no color, and
the colored BGP nexthop contexts are parsed, only if there
is a non colored BGP nexthop context. The actual setup shows
this may not be the case every time.

Fix this by parsing all the colored BGP nexthop contexts.

Fixes: b8210849b8ac ("bgpd: Make bgp ready to remove distinction between 2 nh tracking types")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>