lib,sharpd,pbrd: `set installable` nhg command

lib,sharpd,pbrd: `set installable` nhg command

Add a command `set installable` that allows configured nexthop
groups to be treated as separate/installable objects in the RIB.

A callback needs to be implemented per daemon to handle installing
the NHG into the rib via zapi when this command is set. This
patch includes the implementation for sharpd.

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