zebra: Only install a minimal amount of times

zebra: Only install a minimal amount of times

The code was installing the nexthop group again using
the NLM_F_REPLACE function causing extremely large
route installation times.  This reduces the time from
installing 1 million routes from sharpd with a nhg
from > 200 seconds ( where I gave up ) to ~15
seconds on my machine for 32 x ecmp.  As a side note 1 million
routes using master sharpd takes ~50 seconds to do
the same thing.

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