Implement horizon_images_allow_location mine function

Exposes glance show_multiple_locations option into horizon

Change-Id: I177de06ebf4069a43a012db6f3956dcb27d7db62
Related-Prod: PROD-21005
diff --git a/README.rst b/README.rst
index f050338..a1478c4 100644
--- a/README.rst
+++ b/README.rst
@@ -466,6 +466,17 @@
             strategy: ENCRYPT
             secret_key: secret
 
+Show all image locations when returning an image. This configuration option indicates
+whether to show all the image locations when returning image details to the user.
+
+.. code-block:: yaml
+
+    glance:
+      server:
+        enabled: true
+        ...
+        show_multiple_locations: True
+
 Usage
 =====