[prometheus-nginx] Ensure that nginx is installed, e.g. in root playbook

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

roles/prometheus-nginx-exporter/tasks/main.yml

@@ -2,7 +2,9 @@
 - name: Install prometheus-nginx-exporter
   apt:
     update_cache: true
-    name: prometheus-nginx-exporter
+    name:
+      - nginx  # Nginx may be not already installed
+      - prometheus-nginx-exporter
   register: apt_result
   retries: 3
   until: apt_result is succeeded