undercloud-setup: Introduce tripleo_set_unique_hostname variable

In hostname playbook, add a task to modify /etc/hosts.
tripleo_set_unique_hostname is a new boolean which if set to true,
will add a line to /etc/hosts with an unique hostname
( {{ ansible_hostname }}-unique ) which will resolve the first IPv4
address found in the inventory.

It is useful to reproduce a deployment where the operator is using
hostnames to create SSL certificates and deploy the undecloud with the
public host being an hostname/fqdn and not an IP address.

Related-Bug: #1763776
Change-Id: I6425499e5d248e6cb2285f14dd9c6e716122b90b

roles/undercloud-setup/defaults/main.yml

@@ -41,3 +41,4 @@ undercloud_ara_version: 0.15.0
 undercloud_ara: false
 install_atop: true
 atop_command: atop -R -w /var/log/atop.bin
+tripleo_set_unique_hostname: false

roles/undercloud-setup/tasks/hostname.yml

@@ -16,6 +16,16 @@
         {{ working_dir }}/hostname.sh 2>&1 {{ timestamper_cmd }} > {{ working_dir }}/hostname.sh.log
       become: true
 
+# https://launchpad.net/bugs/1763776
+- name: Add a unique hostname to /etc/hosts resolved to the first IPv4 available
+  become: true
+  lineinfile:
+    dest: /etc/hosts
+    state: present
+    insertafter: EOF
+    line: "{{ ansible_all_ipv4_addresses | first }} {{ ansible_hostname }}-unique"
+  when: tripleo_set_unique_hostname|default(false)|bool
+
 # for undercloud containers only, starting from Rocky.
 - name: Override containerized undercloud CI hostname for tripleo client
   when: