Set the hostname identical to the inventory hostname if no 'hostname' variable has been defined.

library/roles/deb-set-hostname/tasks/main.yml

@@ -1,9 +1,14 @@
 ---
-- name: Set the hostname
+- name: Set the hostname when different from the inventory one.
   hostname: name={{ hostname }}
   when: hostname is defined
   tags: [ 'bootstrap', 'set_hostname' ]
 
+- name: Set the hostname as defined in the inventory
+  hostname: name={{ inventory_hostname  }}
+  when: hostname is not defined
+  tags: [ 'bootstrap', 'set_hostname' ]
+
 - name: Add the hostname to /etc/hosts
   shell: grep -v {{ ansible_default_ipv4.address }} /etc/hosts > /etc/hosts.tmp ; echo "{{ ansible_default_ipv4.address }} {{ hostname }} {{ ansible_hostname }}" >> /etc/hosts.tmp ; /bin/mv /etc/hosts.tmp /etc/hosts
   when: