[constellation] Install docutils for django-admin documentation

Signed-off-by: Yohann D'ANELLO <ynerant@crans.org>

roles/constellation-front/tasks/main.yml

 ---
+- name: Install some front packages
+  apt:
+    install_recommends: false
+    update_cache: true
+    name:
+      python3-docutils
+  register: apt_result
+  retries: 3
+  until: apt_result is succeeded
+
 - name: Install uWSGI
   apt:
     install_recommends: false