Add setting to allow returning the Server header from the backend

[aledbf]

fixes #782

curl -v http://192.168.99.100 -k
* Rebuilt URL to: http://192.168.99.100/
*   Trying 192.168.99.100...
* TCP_NODELAY set
> GET / HTTP/1.1
> Host: 192.168.99.100
> User-Agent: curl/7.52.1
> Accept: */*
> 
< HTTP/1.1 200 OK
< Date: Sun, 28 May 2017 21:46:50 GMT
< Content-Type: text/plain
< Transfer-Encoding: chunked
< Connection: keep-alive
**< Server: echoserver** 
< Strict-Transport-Security: max-age=15724800; includeSubDomains;
< 


Hostname: echoheaders-vt1vr

Pod Information:
	node name:	 minikube
	pod name:	 echoheaders-vt1vr
	pod namespace:	 default
	pod IP:  	 172.17.0.8

Server values:
	server_version=nginx: 1.13.0 - lua: 10008

Request Information:
	client_address=172.17.0.4
	method=GET
	real path=/
	query=
	request_version=1.1
	request_uri=http://192.168.99.100:8080/

Request Headers:
	accept=*/*
	connection=close
	host=192.168.99.100
	user-agent=curl/7.52.1
	x-forwarded-for=192.168.99.1
	x-forwarded-host=192.168.99.100
	x-forwarded-port=443
	x-forwarded-proto=https
	x-original-uri=/
	x-real-ip=192.168.99.1192.168.99.1
	x-scheme=https

Request Body:
	-no body in request-

* Curl_http_done: called premature == 0
* Connection #0 to host 192.168.99.100 left intact

[k8s-reviewable]

This change is 

[coveralls]

Coverage increased (+0.008%) to 46.567% when pulling 2f20c6b on aledbf:pass-server-header into 3a9e3fd on kubernetes:master.