lib: default nexthop weights to one

lib: default nexthop weights to one

Default all nexthop weights to one. The linux kernel does
some weird stuff where it adds one to all nexthop weight values
it gets. So, we added df7fb5800b3798057747873c8be245eb13f3ec36 with
some special subtracing/adding to account for this. Though, that patch
did not account for the default case of the weight being zero for
elements in the group.

Hence, this patch defaults the nexthop weight to one during creation.
This should be a valid value on all platforms anyway so shouldn't
affect anything.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>