Zebra: Fix static NHT to work properly in a VRF

Zebra: Fix static NHT to work properly in a VRF

Implement VRF support for static nexthop resolution (NHT). This is
achieved by ensuring the correct VRF is passed as a parameter to
the NHT functions and is stored in the registered nexthop data
structure.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>
Ticket: CM-9457
Reviewed By: CCR-4185
Testing Done: Manual verification