gce/coreos: Fix dnsmasq image name

This bug was inadvertently introduced in #32406.

The longer term plan (shouldn't be too much longer) is to remove this
file entirely and rely on the `gci-trusty` version of it, but to stop
some bleeding and allow our jenkins using kube-up + coreos to work, we
should merge this fix until we have the more complete solution.

cluster/gce/coreos/kube-manifests/addons/dns/skydns-rc.yaml

@@ -64,7 +64,7 @@ spec:
           name: dns-tcp-local
           protocol: TCP
       - name: dnsmasq
-        image: gcr.io/google_containers/dnsmasq:1.3
+        image: gcr.io/google_containers/kube-dnsmasq-amd64:1.3
         livenessProbe:
           httpGet:
             path: /healthz-dnsmasq