Added annotation for upstream-vhost (#1953)

#1950

docs/user-guide/annotations.md

@@ -50,6 +50,7 @@ The following annotations are supported:
 |[nginx.ingress.kubernetes.io/upstream-max-fails](#custom-nginx-upstream-checks)|number|
 |[nginx.ingress.kubernetes.io/upstream-fail-timeout](#custom-nginx-upstream-checks)|number|
 |[nginx.ingress.kubernetes.io/upstream-hash-by](#custom-nginx-upstream-hashing)|string|
+|[nginx.ingress.kubernetes.io/upstream-vhost](#custom-nginx-upstream-vhost)|string|
 |[nginx.ingress.kubernetes.io/whitelist-source-range](#whitelist-source-range)|CIDR|
 
 **Note:** all the values must be a string. In case of booleans or number it must be quoted.
@@ -124,6 +125,10 @@ To enable consistent hashing for a backend:
 
 `nginx.ingress.kubernetes.io/upstream-hash-by`: the nginx variable, text value or any combination thereof to use for consistent hashing. For example `nginx.ingress.kubernetes.io/upstream-hash-by: "$request_uri"` to consistently hash upstream requests by the current request URI.
 
+### Custom NGINX upstream vhost
+
+This configuration setting allows you to control the value for host in the following statement: `proxy_set_header Host $host`, which forms part of the location block.  This is useful if you need to call the upstream server by something other than `$host`.
+
 ### Certificate Authentication
 
 It's possible to enable Certificate-Based Authentication (Mutual Authentication) using additional annotations in Ingress Rule.