zebra: Remove STATIC_XXX_IFNAME and use _IFINDEX

zebra: Remove STATIC_XXX_IFNAME and use _IFINDEX

When we get a static route through an interface convert the interface
name to an ifindex and pass it through to zebra_rib.c. zebra_rib.c
should not care about the ifname.

This code change will allow us to collapse some of the NEXTHOP_XXX types.

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