Create the grammar for the Nginx upstream module embedded variables

[Micheletto]

See http://nginx.org/en/docs/http/ngx_http_upstream_module.html

[rafrombrc]

Further clarification: The upstream module introduces new embedded variables which can be used in nginx's log_format specification, see http://nginx.org/en/docs/http/ngx_http_upstream_module.html#variables. The request is that the nginx parsing grammar to grow support for these variables.

[trink]

dev now pulls in these updates