Rename Undercloud->External deployment

These plays are better named External deployment instead of Undercloud
deployment, as they aren't actually deploying the Undercloud. It's
confusing to see "Undercloud deployment" and "Overcloud deployment"
tasks in the same output when the undercloud is already deployed.

As the tasks are driving external deployments, and that's what the Heat
output is called (external_deploy_steps_tasks), this renames them to
External deployment.

Change-Id: I685d16d8b3dc5e0d59955ae4c8fac7410168d083

common/deploy-steps.j2

@@ -386,7 +386,7 @@ outputs:
 {%- endfor %}
 {%- for step in range(1,deploy_steps_max) %}
             - hosts: undercloud
-              name: Undercloud deployment step {{step}}
+              name: External deployment step {{step}}
               gather_facts: no
               any_errors_fatal: yes
               become: false