Add an image pull to manage-projects playbook

When we run manage-projects, we always want the latest image.

Add a task file to the gerrit role so that it can have access
to role default variables. Then call it from the playbook
with tasks_from.

Change-Id: I08bd10539d7f08e8759f1c22d66dea18821c9d42

playbooks/manage-projects.yaml

@@ -20,4 +20,6 @@
   name: "Create repos on gerrit servers"
   tasks:
     - name: Run manage-projects
-      command: /usr/local/bin/manage-projects
+      include_role:
+        name: gerrit
+        tasks_from: manage-projects

playbooks/roles/gerrit/tasks/manage-projects.yaml

@@ -0,0 +1,5 @@
+- name: Pull docker image
+  command: "docker pull {{ gerrit_container_image }}"
+
+- name: Run manage-projects
+  command: /usr/local/bin/manage-projects