User correct prefix for docker upper constraints variable

ansible/docker.yml

@@ -3,6 +3,8 @@
   hosts: docker
   tags:
     - docker
+  vars:
+    - docker_upper_constraints_file: "{{ kolla_upper_constraints_file }}"
   roles:
     - role: docker
       docker_daemon_mtu: "{{ public_net_name | net_mtu | default }}"

ansible/roles/docker/defaults/main.yml

@@ -33,3 +33,7 @@ docker_registry_ca:
 
 # MTU to pass through to containers not using net=host
 docker_daemon_mtu: 1500
+
+# Upper constraints file which is passed to pip when installing packages
+# into a venv.
+docker_upper_constraints_file:

ansible/roles/docker/tasks/main.yml

@@ -18,7 +18,7 @@
   pip:
     name: docker
     state: latest
-    extra_args: "-c {{ kolla_upper_constraints_file }}"
+    extra_args: "-c {{ docker_upper_constraints_file }}"
     virtualenv: "{{ virtualenv is defined | ternary(virtualenv, omit) }}"
   become: "{{ virtualenv is not defined }}"