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

Change-Id: If8b6a23fd48b2050f3dc64d55427da150268d5cb
diff --git a/roles/create-vms/files/docker-install-playbook.yml b/roles/create-vms/files/docker-install-playbook.yml
index 032d886..484f196 100644
--- a/roles/create-vms/files/docker-install-playbook.yml
+++ b/roles/create-vms/files/docker-install-playbook.yml
@@ -25,7 +25,7 @@
 
     - name: Add docker apt repo
       apt_repository:
-        repo="deb https://apt.dockerproject.org/repo ubuntu-trusty main"
+        repo: "{{ docker_apt_repo | default('deb https://apt.dockerproject.org/repo ubuntu-trusty main') }}"
 
     - name: Install docker
       apt: