Add a run playbook to propose-updates

By adding a run here, we can easily make jobs either as children or
variants since the only thing needed is the argument.

Change-Id: I6d19b8d9fb96210b8b92103c095fd8c8b9c8309b

playbooks/proposal/propose-updates.yaml

@@ -0,0 +1,7 @@
+- hosts: all
+  tasks:
+
+    - name: Run propose_update.sh script
+      command: "{{ ansible_user_dir }}/scripts/propose_update.sh {{ update_target }}"
+      args:
+        chdir: "src/{{ zuul.project.canonical_name }}"

zuul.yaml

@@ -190,6 +190,7 @@
 - job:
     name: propose-updates
     pre-run: playbooks/proposal/pre
+    run: playbooks/propose-updates
     secrets:
       - proposal_ssh_key