debian: allow overriding release

roles/debian/handlers/main.yml

@@ -9,3 +9,6 @@
 
 - name: restart frr
   service: name=frr state=restarted
+
+- name: update package cache
+  apt: update_cache=true

roles/debian/tasks/main.yml

@@ -24,6 +24,16 @@
     src: resolv.conf.j2
     mode: 0644
 
+- name: Set up debian repositories
+  template:
+    dest: /etc/apt/sources.list
+    src: sources.list.j2
+    mode: 0644
+  notify: update package cache
+  when: debian_release is defined
+
+- meta: flush_handlers
+
 - name: Install essential packages
   package: name=git,rsync,vim,tmux