lib, zebra: Abstract vrf.c to handle both vrf_id_t and char *name

lib, zebra: Abstract vrf.c to handle both vrf_id_t and char *name

Abstract vrf.c code to allow pass in a vrf name as well as to
start the coding of how namespaces and vrf's will interact.

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