CORD-536 updated to support the setting of repo via vars

Change-Id: I1d9a555773a29ba6e000cbd31f0e08bf23dc8622
diff --git a/ansible/roles/docker/tasks/debian.yml b/ansible/roles/docker/tasks/debian.yml
index f590f17..94188b1 100644
--- a/ansible/roles/docker/tasks/debian.yml
+++ b/ansible/roles/docker/tasks/debian.yml
@@ -1,6 +1,6 @@
 - name: Debian add Docker repository and update apt cache
   apt_repository:
-    repo: deb https://apt.dockerproject.org/repo ubuntu-{{ debian_version }} main
+    repo: "{{ docker_apt_repo | default('deb https://apt.dockerproject.org/repo ubuntu-{{ debian_version }} main') }}"
     update_cache: yes
     state: present
   tags: [docker]