Update horizon config options

Set horizon option below in the following order:
- IMAGES_ALLOW_LOCATION

1. From pillar data:
horizon:server:images_allow_location

2 From glance mine function:
set the IMAGES_ALLOW_LOCATION according
to the glance:server:show_multiple_locations pillar data

3. Default
IMAGES_ALLOW_LOCATION = True

Change-Id: If613cff0286f8cf784112fdcdb0946729924003e
Related-Prod: PROD-21005
diff --git a/README.rst b/README.rst
index 4f42745..f9ead07 100644
--- a/README.rst
+++ b/README.rst
@@ -564,7 +564,7 @@
 Images upload mode:
 Horizon allows to use different strategies when uploading images to glance that are
 controlled by `horizon:server:images_upload_mode` pillar, possible options are
-direct, ligacy, off. When `direct` mode is used CORS have to be disabled on glance
+direct, ligacy, off. When `direct` mode is used CORS have to be enabled on glance
 side, and client should use modern browser.
 
 .. code-block:: yaml
@@ -573,6 +573,19 @@
       server:
         images_upload_mode: "direct"
 
+Images allow location:
+If set to True, this setting allows users to specify an image location (URL) as the image source
+when creating or updating images. Depending on the Glance version, the ability to set an image
+location is controlled by policies and/or the Glance configuration. Therefore IMAGES_ALLOW_LOCATION
+should only be set to True if Glance is configured to allow specifying a location.
+
+.. code-block:: yaml
+
+    horizon:
+      server:
+        images_allow_location: True
+
+
 Upgrades
 ========