Only force apt backports for Trusty

The backports repo is only essential for Trusty at this time,
so this patch ensures that this task is only applied to Trusty.

Change-Id: I5026b66a0b263b56d604b06b8efea2eba80447d0

common-tasks/test-force-package-cache-update.yml

@@ -19,6 +19,7 @@
   command: "sed -i '/deb.*-backports/s|^#\\s||g' /etc/apt/sources.list"
   when:
     - ansible_pkg_mgr == 'apt'
+    - ansible_distribution_release | lower == 'trusty'
 
 - name: First ensure apt cache is always refreshed
   apt: