zebra: don't add ID to kernel nh_grp if not installed/queued

zebra: don't add ID to kernel nh_grp if not installed/queued

When we transform the nexthop group rb trees into a flat
array of IDs to send into the dataplane code (zebra_nhg_nhe2grp),
don't put an ID in there that has not been in installed or is
not currently queued to be installed into the dataplane.

Otherwise, if some of the nexthops fail to install, we will
still try to create a group with them and then the entire group
will fail.

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