Add nginx rate limiting

[scottlinux]

To prevent unintended flooding of requests which can impact a jenkins master, nginx rate limiting can be used.

Random example for the @ jenkins location block of cinch:

roles/jenkins_master/templates/etc/nginx/conf.d/jenkins_https.conf

limit_req_zone $binary_remote_addr zone=jenkins:10m rate=25r/s;

server {

...

  location @jenkins {
    proxy_http_version  1.1;
    proxy_connect_timeout       30s;
    proxy_send_timeout          120;
    proxy_read_timeout          120;
    client_max_body_size 35m;
    limit_req zone=jenkins;
    proxy_buffer_size 4k;
    proxy_buffers 8 32m;
    proxy_set_header        Host $host;
    proxy_set_header        X-Real-IP $remote_addr;
    proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header        X-Forwarded-Proto $scheme;
    proxy_redirect http:// https://;
    proxy_pass              http://jenkins;
  }

https://www.nginx.com/blog/rate-limiting-nginx/

[scottlinux]

A person in #nginx on freenode suggested also looking into bandwidth rate limiting which can be implemented conditionally, if desired.

http://nginx.org/en/docs/http/ngx_http_core_module.html#limit_rate

https://www.nginx.com/resources/admin-guide/restricting-access/

I'll test some things out offline then work up a PR.

[scottlinux]

I have some working configs that seem plausible. I will work up a PR.

[scottlinux]

I have some updated changes to make this an optional feature to enable. Feature would be disabled by default. PR will be updated with changes.

[scottlinux]

This was merged in as an opt-in feature for cinch.

#188