zebra: northbound changes for the rib model

zebra: northbound changes for the rib model

This commit implements:
RIB operational list create/destroy.
Walk over RIB tables using keys.
The first RIB table will be IPV4/unicast (table-id 254)
will be fetched.
Create a new api to fetch RIB table based on
afi-safi and table id as the keys.

remove mandatory true statement from the leaf which
is part of the list key.

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>