use a bit of yaml magic to de-duplicate the users' extra_groups

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

group_vars/all.yml

 ---
 
+_admin_groups: &_admin_groups
+  - allow-ssh
+  - passwordless-sudo
+
 handled_users:
   - name: manager
     gecos: Management user
     # the keys of this user are handled automatically by the 'admin_users' host var
     keys: []
     home: /home/manager
-    extra_groups:
-      - allow-ssh
-      - passwordless-sudo
+    extra_groups: *_admin_groups
 
   - name: mattia
     gecos: Mattia Rizzolo
@@ -17,9 +19,7 @@ handled_users:
         comment: mattia@4B043FCDB9444540
         state: present
     shell: /bin/zsh
-    extra_groups:
-      - allow-ssh
-      - passwordless-sudo
+    extra_groups: *_admin_groups
 
   - name: pietro
     gecos: Pietro Albini
@@ -30,9 +30,7 @@ handled_users:
       - key: ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCdVVy/lfTMwDVTDXFJmF/BzI7Rjw3Fu4WkVnkdJnAHcBEZk/hOAoBq26mNzcX3VbLlpSd48UrtR8gEKqmeHwBeMkODgui+XC3TszRxbTVzr0f4bcnSj/XNfptXzbXfyf2h/PxrCrgE5E2JD3O4BAcGr3QGeF6fP7Q9J9pbLIHZVvoiX4/Ll7NLJtQrKa5ssBD4kpYt3HS6Ab25lWvE0PLu4J3VpM29DBmVxp3cBnbENlr9hh2XDhgyKwjkboKiQkK1s1/pfFDOhI4UYVu2g3Jzwxk+KxGgsLxenADtdENZRsinfDFzxrhHmpLhrV+e3pTpvwnEO3AaS/WWdxSgBBYf
         comment: backup-pietro
         state: present
-    extra_groups:
-      - allow-ssh
-      - passwordless-sudo
+    extra_groups: *_admin_groups
 
   - name: janvitus
     gecos: Gianvito Cavasoli
@@ -40,9 +38,7 @@ handled_users:
       - key: ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDUzuSDyKJeIbGTGk6CeoxBZGGSJMr03O7TScBjzD45E98ejxknQoFqsGeoi6rSo8YuvSHRVO3rhCwpcnIjIkZM0BEAwPilUrxiuIXBMkuu5Rv/0CB8PsVP9kg9XtmJr2PaA0iQKg4ije0CEG5j/pom9QZsaWggCIp+SAoIFys7G1bSRmPLHghBWxpoKtd1pjhbwZJ+e/kAPIWaDZcILvTVqdUn8WpzbA0mqXWdSwXmf2fggx+u0Qj1PEJWJnmQgf3TAlnBfUHcYsjdFp2jnEzM+kFhXqXB17FijZn3Uz5R2VFsLUsX10BxV6frKOi536/KvXlbwhoEwIBtg2aHX7/F
         comment: gianvito@ubuntubox
         state: present
-    extra_groups:
-      - allow-ssh
-      - passwordless-sudo
+    extra_groups: *_admin_groups
 
 handled_groups:
   - name: allow-ssh