bfdd: enable bfd session if vrf interface available

bfdd: enable bfd session if vrf interface available

The vrf interface notification and interface notifications are separated
on zapi interface between the system (zebra daemon) and other daemons
(bfd for instance). In the case of bfd, the initial code was waiting for
vrf notification to create the socket. Actually, in vrf-lite world, we
need to wait the vrf interface to be present, in order to create the
socket and bind to the vrf interface (this is the usual way to work with
vrf-lite).
On bfd, the changes consist in delaying the socket creation first, then
when interface is created, check the interface name presence instead of
checking the interface configuration.

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