Adding test_update_router_admin_state to skiplist

This test is failing from time to time due bug #1921400

Change-Id: Iffce3b4cd42f3f0da1f096727aead46a66af9d2e

roles/validate-tempest/vars/tempest_skip.yml

@@ -1774,6 +1774,16 @@ known_failures:
       - name: wallaby
         reason: auth issues
         lp: https://bugs.launchpad.net/tripleo/+bug/1921400
+  - test: tempest.scenario.test_network_basic_ops.TestNetworkBasicOps.test_update_router_admin_state
+    deployment:
+      - overcloud
+    releases:
+      - name: master
+        reason: auth issues
+        lp: https://bugs.launchpad.net/tripleo/+bug/1921400
+      - name: wallaby
+        reason: auth issues
+        lp: https://bugs.launchpad.net/tripleo/+bug/1921400
   - test: tempest.scenario.test_network_basic_ops.TestNetworkBasicOps.test_connectivity_between_vms_on_different_networks
     deployment:
       - overcloud