addons: vrf: add a check and error for reserved system table names

addons: vrf: add a check and error for reserved system table names

Ticket:
Reviewed By: dsa, nikhil, julien
Testing Done: Tested with system reserved table names

example: err msg:
$ifquery default
auto default
iface default
	vrf-table auto

error: default: cannot use system reserved ['253', '0', '254',
'255'] table names

This patch makes all tables in  /etc/iproute2/rt_tables reserved

$cat /etc/iproute2/rt_tables
255	local
254	main
253	default
0	unspec

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>