letsencrypt-install-txt-record: skip disabled hosts

We are seeing:

  fatal: [adns1.opendev.org]: FAILED! => {"msg": "The task includes an
  option with an undefined variable. The error was:
  'ansible.vars.hostvars.HostVarsVars object' has no attribute
  'acme_txt_required'

I belive this is because we have a disabled mirror host now.  So the
iad.rx.opendev.org mirror is in the "letsencrypt" group, but because
it is also disabled the prior role (letsencrypt-request-certs) has not
run and it has not populated it's "acme_txt_required" variable.

We should skip disabled hosts when inspecting the hosts for this
variable.  Add this to the "with_inventory_hostnames" match.

Change-Id: I33a1c8b6f7e8499248e370f69a9f573a2bf106a5

playbooks/roles/letsencrypt-install-txt-record/tasks/main.yaml

@@ -5,7 +5,8 @@
 - name: Build key list
   set_fact:
     acme_txt_keys: '{{ acme_txt_keys }} + {{ hostvars[item]["acme_txt_required"] }}'
-  with_inventory_hostnames: letsencrypt
+  with_inventory_hostnames:
+    - letsencrypt:!disabled
 
 - name: Final list
   debug: