roles/common: Adjust cron-apt stuff

- Don't run the static files as templates
- Use a separate playbook for related tasks
- Use a template for security.sources.list

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/common/files/etc/cron-apt/config

@@ -0,0 +1,5 @@
+# Configuration for cron-apt. For further information about the possible
+# configuration settings see the README file.
+
+MAILON="never"
+OPTIONS="-o quiet=1 -o Dir::Etc::SourceList=/etc/apt/security.sources.list -o Dir::Etc::SourceParts=\"/dev/null\""