Unhardcode proxy_set_header

This patch unhardcode proxy_set_header options from proxy template.
The default behaviour is keept. Header might be passed via the following
pillar of site definition:

  nginx:
    server:
      site:
        abc:
          type: nginx_proxy
          proxy_set_header:
            My-Header:
              enabled: true
              value: '"MyValue"'
              
Related-Prod: PROD-23522

Change-Id: I8108b7ca0deab6b0c79a764beec3ca90e39d3f17
diff --git a/README.rst b/README.rst
index 8c08775..927eaaf 100644
--- a/README.rst
+++ b/README.rst
@@ -472,6 +472,33 @@
                 always: true
                 enabled: true
 
+Setting custom proxy headers:
+
+.. code-block:: yaml
+
+    nginx:
+      server:
+        enabled: true
+        site:
+          custom_headers:
+            type: nginx_proxy
+            proxy_set_header:
+              Host:
+                enabled: true
+                value: "$host:8774"
+              X-Real-IP:
+                enabled: true
+                value: '$remote_addr'
+              X-Forwarded-For:
+                enabled: true
+                value: '$proxy_add_x_forwarded_for'
+              X-Forwarded-Proto:
+                enabled: true
+                value: '$scheme'
+              X-Forwarded-Port:
+                enabled: true
+                value: '$server_port'
+
 Nginx stats server (required by collectd nginx plugin):
 
 .. code-block:: yaml