Add ability to override nginx ssl engine as not defined

If by default engine set to 'salt' and there is no way to unset it.

Change-Id: Iaac14c2afc9a8604642145ddbc0e0aacb8ca6b46
Related-Prod: PROD-27578
diff --git a/README.rst b/README.rst
index fa005ef..630b919 100644
--- a/README.rst
+++ b/README.rst
@@ -531,6 +531,21 @@
               key_file: /etc/ssl/private/mykey.key
               cert_file: /etc/ssl/cert/mycert.crt
 
+or
+
+.. code-block:: yaml
+
+    nginx:
+      server:
+        enabled: true
+        site:
+          mysite:
+            ssl:
+              enabled: true
+              engine: custom
+              key_file: /etc/ssl/private/mykey.key
+              cert_file: /etc/ssl/cert/mycert.crt
+
 Advanced SSL configuration, more information about SSL option
 may be found at http://nginx.org/en/docs/http/ngx_http_ssl_module.html