docker-host: use apt preferences and apt role

* the "apt" role can deal with backports lists install
* we'd rather use a preferences file for specific packages
  than simply installing jessie-backports

docker-host/files/docker_preferences

@@ -0,0 +1,3 @@
+Package: python-docker
+Pin: release a=jessie-backports
+Pin-Priority: 999

docker-host/tasks/jessie_backports.yml

@@ -0,0 +1,23 @@
+---
+- include_role:
+    name: apt
+    tasks_from: backports.yml
+  tags:
+  - packages
+
+- name: Prefer python-docker package from jessie-backports
+  copy:
+    src: apt/docker_preferences
+    dest: /etc/apt/preferences.d/999-docker
+    force: yes
+    mode: "0640"
+  register: docker_apt_preferences
+  tags:
+  - packages
+
+- name: update apt
+  apt:
+    update_cache: yes
+  when: docker_apt_preferences | changed
+  tags:
+  - packages

docker-host/tasks/main.yml

@@ -24,11 +24,9 @@
     repo: 'deb [arch=amd64] https://download.docker.com/linux/debian {{ ansible_distribution_release }} stable'
     state: present
     update_cache: no
+    filename: docker.list
 
-- name: Enable backports repository for python-docker (Jessie only)
-  apt_repository:
-    repo: 'deb http://ftp.debian.org/debian {{ ansible_distribution_release }}-backports main'
-    state: present
+- include: jessie_backports.yml
   when: ansible_distribution_release == 'jessie'
 
 - name: Add Docker's official GPG key