r/nginx: Configure logrotate

The default `logrotate` configuration for _nginx_ may not be appropriate
for high-volume servers.  The `nginx_keep_num_logs` variable is now
available to control how many days of logs are kept.

roles/nginx/tasks/main.yml

@@ -91,3 +91,13 @@
   service:
     name: nginx
     enabled: yes
+
+- name: ensure logrotate is configured for nginx
+  template:
+    src: nginx.logrotate.j2
+    dest: /etc/logrotate.d/nginx
+    mode: u=rw,go=r
+    owner: root
+    group: root
+  tags:
+  - logrotate