Force install of specified docker version, fixes #295

This allows Ubuntu/Debian to downgrade Docker version if
a newer version is installed, instead of failing.

roles/docker/tasks/main.yml

@@ -48,7 +48,8 @@
 - name: ensure docker packages are installed
   action: "{{ docker_package_info.pkg_mgr }}"
   args:
-    pkg: "{{item}}"
+    pkg: "{{item.name}}"
+    force: "{{item.force|default(omit)}}"
     state: present
   with_items: "{{ docker_package_info.pkgs }}"
   when: (ansible_os_family != "CoreOS") and (docker_package_info.pkgs|length > 0)