zebra: handle NHG in NHG dataplane group conversion

zebra: handle NHG in NHG dataplane group conversion

We were not properly handling the case of a NHG inside of
another NHG when converting the rb tree of a multilevel NHG
into a flat list of IDs. When constructing, we call the function
zebra_nhg_nhe2grp_internal() recursively so that the rare
case of a group within a group is handled such that its
singleton nexthops are appended to the grp array of IDs
we send to the dataplane code.

Ex)

1:
	-> 2:
		-> 3
		-> 4
	->5:
		->6

becomes this:

1:
	->3
	->4
	->6

when its sent to the dataplane code for final kernel installation.

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