bgpd: add an API to get incoming VRF from a RT

bgpd: add an API to get incoming VRF from a RT

This commit is relying on bgp vpn-policy. It is needed to configure
several bgp vrf instances, and in each of the bgp instance, configure
the following command under address-family ipv4 unicast node:
[no] rt redirect import RTLIST

Then, a function is provided, that will parse the BGP instances.
The incoming ecommunity will be compared with the configured rt redirect
import ecommunity list, and return the VRF first instance of the matching
route target.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>