zebra: nexthop groups vrf's are only a function of namespaces

zebra: nexthop groups vrf's are only a function of namespaces

Nexthop groups as a whole do not make sense to have a vrf'ness
As that you can have a arbitrary number of nexthops that point
to separate vrf's.

Modify the code to make this distinction, by clearly delineating
the line between the nhg and the nexthop a bit better.
Nexthop groups having a vrf_id only make sense if you are using
network namespaces to represent them.

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