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/templates/nginx.logrotate.j2

@@ -0,0 +1,13 @@
+/var/log/nginx/*.log {
+    create 0640 nginx root
+    daily
+    rotate {{ nginx_keep_num_logs }}
+    missingok
+    notifempty
+    compress
+    delaycompress
+    sharedscripts
+    postrotate
+        /bin/kill -USR1 `cat /run/nginx.pid 2>/dev/null` 2>/dev/null || true
+    endscript
+}