k8s:UNable to reach public /outisde netowrk unless custome tags are associated with __public__ netwrok

In case of the logical router enabled with SNAT ,pod reachability to the external networks(outside the cluster) is dropped unless we associate the custom tag to the __public__ vn

due to this, testcase :TestPod.test_pod_public_reachability_using_snat fails .

since association of the custom tag solving the issue ..hence not marking it as sanity blocker .

Build :5.0.263
Deployment :Ansible_deployer
HOST OS: CENTOS7.5
=======================

Topology
==================
vrouter +k8s_node:

      ip: nodec60
      ip: nodec61

config +control++kubemanager:

      ip: nodeg12(k8s_master)
      ip: nodeg31
      ip: nodec58

if __public__ is associated with custome tag namespace=default ...then it works fine .

letftnet(100.64.0.0/29) rightnet __public__(10.204.221.160/29)
pod -----------------SNATroutr------------------------ 10.84.5.120
10.47.255.251

[root@nodeg12 ~]# kubectl get pods --all-namespaces -o wide
NAMESPACE NAME READY STATUS RESTARTS AGE IP NODE
ctest-namespace-94298939 ctest-ubuntuapp-pod-60066249 1/1 Running 0 20m 10.47.255.251 nodec61

   434368<=>9744 10.47.255.251:204 1 (2)
                         10.84.5.120:0
(Gen: 1, K(nh):21, Action:F, Flags:, QOS:-1, S(nh):21, Stats:1/98, SPort 65505,
 TTL 0, Sinfo 3.0.0.0)

   359936<=>405536 10.84.5.120:186 1 (4)
                         10.47.255.251:0
(Gen: 1, K(nh):37, Action:F, Flags:, QOS:-1, S(nh):37, Stats:0/0, SPort 54224,
 TTL 0, Sinfo 0.0.0.0)

    86460<=>95204 10.204.221.164:132 1 (3)
                         10.84.5.120:0
(Gen: 1, K(nh):36, Action:D(FwPolicy), Flags:, QOS:-1, S(nh):36, Stats:3/294,
 SPort 61117, TTL 0, Sinfo 5.0.0.0)

    95204<=>86460 10.84.5.120:132 1 (3)
                         10.204.221.164:0
(Gen: 1, K(nh):36, Action:D(Unknown), Flags:, QOS:-1, S(nh):34, Stats:0/0,
 SPort 63555, TTL 0, Sinfo 0.0.0.0)