lib, sharpd, zebra: Update the zapi_vrf_label call to add afi

lib, sharpd, zebra: Update the zapi_vrf_label call to add afi

Add the ability to pass in an afi to zebra.  zebra_vrf keeps
track of the afi/label tuple and then does the right thing
before we call down.  AF_MPLS does not care about v4 or v6
it just knows label and what device to use for lookup.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>