The lookup for non default VRFs was always using a tableId; if not
provided, we were defaulting to RT_TABLE_MAIN. This is fine for the
default VRF but not for others. As a result, the command was silently
failing for non-default VRFs unless we also specified the correct tableId.
Fix this by only performing the lookup using the tableId if it is
provided; else use zebra_vrf_table.
Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>