Add become: true for host_prep_tasks

I561b5ef6dee0ee7cac67ba798eda284fb7f7a8d0 added this for the main
deploy steps, but there are some host_prep_tasks which require this,
specifically the nova-libvirt tasks fail for me locally without -b

Change-Id: I29cb8b0962c0dfcf7950d65305a3adef1f1268c3

common/deploy-steps.j2

@@ -424,6 +424,7 @@ outputs:
               any_errors_fatal: yes
               vars:
                 bootstrap_server_id: BOOTSTRAP_SERVER_ID
+              become: true
               tasks:
 {%- for role in roles %}
                 - include: {{role.name}}/host_prep_tasks.yaml