test: Update coredns image tag from 1.2.2 to 1.2.6 for k8s 1.12

Update k8s 1.12 coredns deployment to image tag 1.2.6 to get the bug
fix for loop detection getting confused due to retries
(coredns/coredns#2391).

Fixes: #6307
Signed-off-by: Jarno Rajahalme <jarno@covalent.io>

test/provision/manifest/1.12/coredns_deployment.yaml

@@ -1,5 +1,7 @@
 # File source
 # https://raw.githubusercontent.com/kubernetes/kubernetes/release-1.12/cluster/addons/dns/coredns/coredns.yaml.base
+# Local changes:
+# 8/13/2020: coredns image updated to 1.2.6 to fix #6307 (jrajahalme)
 ---
 apiVersion: v1
 kind: ServiceAccount
@@ -111,7 +113,7 @@ spec:
         operator: "Exists"
       containers:
       - name: coredns
-        image: k8s.gcr.io/coredns:1.2.2
+        image: k8s.gcr.io/coredns:1.2.6
         imagePullPolicy: IfNotPresent
         resources:
           limits: