sharpd: Allow sharpd to accept nexthop group as part of route install

sharpd: Allow sharpd to accept nexthop group as part of route install

When installing routes via sharpd 'sharp install route... ' command
add the ability to specify a nexthop-group to use.  This will
allow sharpd to create ECMP routes into zebra.

Nexthop-group:
!
nexthop-group JANELLE
  nexthop 192.168.209.1
  nexthop 192.168.210.1
!

The install:

donna.cumulusnetworks.com# sharp install routes 10.0.50.0 nexthop-group JANELLE 10
donna.cumulusnetworks.com# end
donna.cumulusnetworks.com# show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP,
       O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
       T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
       F - PBR, f - OpenFabric,
       > - selected route, * - FIB route

K>* 0.0.0.0/0 [0/106] via 10.0.2.2, enp0s3, 00:20:38
C>* 10.0.2.0/24 is directly connected, enp0s3, 00:20:38
D>* 10.0.50.0/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
D>* 10.0.50.1/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
D>* 10.0.50.2/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
D>* 10.0.50.3/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
D>* 10.0.50.4/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
D>* 10.0.50.5/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
D>* 10.0.50.6/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
D>* 10.0.50.7/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
D>* 10.0.50.8/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
D>* 10.0.50.9/32 [150/0] via 192.168.209.1, enp0s8, 00:00:02
  *                      via 192.168.210.1, enp0s9, 00:00:02
C>* 192.168.209.0/24 is directly connected, enp0s8, 00:20:38
C>* 192.168.210.0/24 is directly connected, enp0s9, 00:20:38
donna.cumulusnetworks.com#

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