Add X-VCAP-Request-ID to nginx access log

Based on a suggestion from @phanle, add vcap request id to the access log for improved correlation between response times and request activity in the cc log.

`10.244.0.138 - [24/Feb/2014:23:18:05 +0000] "GET /v2/info HTTP/1.1" 200 527 "-" "monit/5.2.4" 10.244.0.138 vcap_request:8b9f2ca9-046e-40fa-951b-1ad1fa803374 response_time:0.009`