configure-mirrors: fix stripped newline

I74b9de7092f182c942a58ac7a46b9fbd791889de hit a common ansible gotcha
where it likes to strip the trailing newline after a {% endif %}.
This has resulted in invalid lines in our sources.list.

Unfortunately we miss this because it still exits with 0.  Add a
simple test looking for warning output.

Change-Id: I46d393a5e67d10a52c4dcca803176ff368a4b5bd

roles/configure-mirrors/templates/apt/etc/apt/sources.list.j2

@@ -1,3 +1,4 @@
+#jinja2: trim_blocks:False
 # {{ ansible_managed }}
 deb {% if set_apt_mirrors_trusted %}[ trusted=yes ] {% endif %}{{ package_mirror }} {{ ansible_distribution_release }} main universe
 deb {% if set_apt_mirrors_trusted %}[ trusted=yes ] {% endif %}{{ package_mirror }} {{ ansible_distribution_release }}-updates main universe

test-playbooks/base-roles/configure-mirrors.yaml

@@ -37,3 +37,19 @@
         name: "{{ emacs_package | default('emacs') }}"
         state: "present"
       become: yes
+
+- name: Ensure no apt warnings
+  hosts: all
+  tasks:
+    - name: Check apt output
+      when: ansible_distribution == 'Debian' or ansible_distribution == 'Ubuntu'
+      block:
+        - name: Run apt-get update
+          command: 'apt-get update'
+          register: _apt_get_output
+          become: yes
+
+        - name: Check for warnings in output
+          fail:
+            msg: 'Warnings found in apt output'
+          when: _apt_get_output is regex('^W:.*$')