lib: handling vrf backend unknown case

lib: handling vrf backend unknown case

on interface search algorithm, at initialisation, when reading config
file, the vrf backend may not be yet known ( because zebra did not sync
yet with daemon). For that, avoid searching interface name in a separate
vrf. This change of behaviour is induced because the assumption is done
that at config startup, the user is not wrong with the interface
configuration to use. Every usage of vrf_get_backend() should then be
wisely adapted in order to handle that init state.

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