zebra: Do not delete nhg's when retain_mode is engaged

zebra: Do not delete nhg's when retain_mode is engaged

When `-r` is specified to zebra, on shutdown we should
not remove any routes from the fib.  This was a problem
with nhg's on shutdown due to their ref-count behavior.

Introduce a methodology where on shutdown we don't mess
with the nexthop groups in the kernel.  That way on
next startup things will be ok.

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