[refactor] use ansible playbook standard layout

Ansible recommend a specific directory layout to deal with complex
playbooks. As we expect this configuration to become trickier, let's
adopt good practices right away.

ansible/main.yaml

 ---
 - hosts: all
-  tasks:
-    - name: update etc/hosts
-      lineinfile:
-        dest: /etc/hosts
-        line: "{{ hostvars[item].ansible_host }} {{ hostvars[item].inventory_hostname }} {{ hostvars[item].inventory_hostname_short }}"
-        state: present
-      with_items: "{{ groups.all }}"
-      become: yes
-
+  roles:
+    - common

ansible/roles/common/tasks/main.yaml

+ - name: update etc/hosts
+   lineinfile:
+     dest: /etc/hosts
+     line: "{{ hostvars[item].ansible_host }} {{ hostvars[item].inventory_hostname }} {{ hostvars[item].inventory_hostname_short }}"
+     state: present
+   with_items: "{{ groups.all }}"
+   become: yes