added check for inband ports in ip route result parse (#4728)

What is the motivation for this PR?
In T2 topology for a VOQ Chassis, if the linecard learns the default route from eBGP, an snmp entry is created in the ipCidrRouteTable
But if the linecard learns the default route through an in-band port (example a linecard connecting to only T1 VM's), there will not be any entry created in the snmp ipCidrRouteTable.

This is per SONiC design as defined in the following code in rfc4292.py

    if (ifn in port_table and
       multi_asic.PORT_ROLE in port_table[ifn] and
       port_table[ifn][multi_asic.PORT_ROLE] == multi_asic.INTERNAL_PORT):
          continue

Thus, the test case fails.

How did you do it?
Added a condition to check if the output from "show ip route " command has internal inband or backplane port in it, and if so don't add it to list of nexthops to check.

How did you verify/test it?
ran test_snmp_default_route on VOQ Chassis

tests/snmp/test_snmp_default_route.py

@@ -28,7 +28,7 @@ def test_snmp_default_route(duthosts, enum_rand_one_per_hwsku_frontend_hostname,
             ip, interface = line.split('via')
             ip = ip.strip("*, ")
             interface = interface.strip("*, ")
-            if interface != "eth0":
+            if interface != "eth0" and 'Ethernet-IB' not in interface and 'Ethernet-BP' not in interface:
                 dut_result_nexthops.append(ip)
 
     # If show ip route 0.0.0.0/0 has route only via eth0,