Add ability to checkout specific revision of nginx-upload-module

There is no build that supports nginx version > 1.3.8. You need to checkout revision with the fix.

    "nginx": {
      "default_site_enabled": false,
      "source": {
        "version": "1.7.3",
        "modules": [
          "nginx::http_gzip_static_module",
          "nginx::http_ssl_module",
          "nginx::upload_progress_module",
          "nginx_upload_module::upload_module"
        ]
      },
      "upload": {
        "strategy": "git",
        "url" : "https://github.com/vkholodkov/nginx-upload-module.git",
        "revision": "ed996c4f7fedd9f2a1cce97f79192cd5c057e797"
      }
    }