r/nginx: Make logging to files optional

If _nginx_ is configured to send error/access log messages to syslog, it
may not make sense to _also_ send messages to log files as well.  The
`nginx_error_log_file` and `nginx_access_log_file` variables are now
available to control whether/where to send log messages.  Setting either
of these to a falsy value will disable logging to a file.  A non-empty
string value is interpreted as the path to a log file.  By default, the
existing behavior of logging to `/var/log/nginx/error.log` and
`/var/log/nginx/access.log` is preserved.

roles/nginx/defaults/main.yml

@@ -4,5 +4,7 @@ nginx_ssl_session_cache: shared:SSL:1m
 nginx_ssl_session_timeout: 10m
 nginx_ssl_ciphers: '{{ nginx_default_ssl_ciphers }}'
 nginx_log_syslog: true
+nginx_error_log_file: /var/log/nginx/error.log
+nginx_access_log_file: /var/log/nginx/access.log
 nginx_redirect_http_https: false
 nginx_keep_num_logs: 10