install-docker: add package version option

Add a variable to allow selecting a specific docker version.  Useful
if a new docker releases but your jobs aren't quite ready.

Change-Id: Ie4c178d966474476fd2de4489738e47e8a641834

roles/install-docker/README.rst

@@ -24,3 +24,11 @@ An ansible role to install docker and configure it to use mirrors if available.
    Which update channel to use for upstream docker. The two choices are
    ``stable``, which is the default and updates quarterly, and ``edge``
    which updates monthly.
+
+.. zuul:rolevar:: docker_version
+   :default: undefined
+
+   Declare this with the version of the docker package to install.
+   Undefined will install the latest.  This will look something like
+   ``18.06.1~ce~3-0~ubuntu``.  Only supported when using upstream
+   docker repos.

roles/install-docker/tasks/upstream.yaml

@@ -27,6 +27,6 @@
 - name: Install docker
   become: yes
   apt:
-    name: docker-ce
+    name: "docker-ce{% if docker_version is defined %}={{ docker_version }}{% endif %}"
     state: present
     update_cache: yes