Apt-cacher-ng's default ubuntu mirror list has faulty mirrors

The top mirror(http://mirror.veracruz.co/ubuntu/) is currently broken.
We can override the defaults by editing the override file
(/etc/apt-cacher-ng/backends_ubuntu) with valid mirrors.

Change-Id: I35f7efa665030282c9630d7423413e8bd3628ba5
Closes-Bug: 1759903

multi-node-aio/playbooks/deploy-acng.yml

@@ -65,6 +65,16 @@
         curl https://www.centos.org/download/full-mirrorlist.csv | sed 's/^.*"http:/http:/' | sed 's/".*$//' | grep ^http >/etc/apt-cacher-ng/centos_mirrors
         echo "http://mirror.centos.org/centos/" >>/etc/apt-cacher-ng/centos_mirrors
 
+    - name: override default ubuntu mirror list
+      copy:
+        dest: /etc/apt-cacher-ng/backends_ubuntu
+        mode: 0644
+        owner: root
+        group: root
+        content: 'http://archive.ubuntu.com/ubuntu/'
+      notify:
+        - reload acng
+
     - name: Drop acng.conf
       template:
         src: "pxe/acng.conf.j2"