Use a single task for fact gathering

We don't need to do different plays for fact gathering as we can simply
include both the source/remote hosts in a single pattern.

For the record, this was added because we were using local connections
for the undercloud when running under mistral however this is no longer
the case. Additionally I believe we stopped doing that later as well but
this may not be able to be backported to run under mistral. Original
context https://review.opendev.org/#/c/518323/

Change-Id: I4298855fe71b1035df81d5ce9c3cef2e3c96dbb6
(cherry picked from commit 27ec857cf2)

common/deploy-steps-playbooks-common.yaml

@@ -10,17 +10,8 @@
   tasks:
     - meta: clear_facts
 
-- hosts: "{{ deploy_source_host }}"
-  name: Gather facts from undercloud
-  gather_facts: yes
-  any_errors_fatal: true
-  ignore_unreachable: false
-  become: false
-  tags:
-    - facts
-
-- hosts: "{{ deploy_target_host }}"
-  name: Gather facts from overcloud
+- hosts: "{{ deploy_source_host }}:{{ deploy_target_host }}"
+  name: Gather facts
   gather_facts: yes
   # False because https://github.com/ansible/ansible/issues/70663
   any_errors_fatal: false