fix: disallow embedded of horizon in iframe (#338)

OpenStack Helm enables the Horizon service to be embedded in an iframe
by default. This is a security risk and should be disabled.

This patch disables the embedding of Horizon in an iframe.
diff --git a/roles/horizon/vars/main.yml b/roles/horizon/vars/main.yml
index 012a539..f7d8c96 100644
--- a/roles/horizon/vars/main.yml
+++ b/roles/horizon/vars/main.yml
@@ -23,6 +23,7 @@
     horizon:
       local_settings:
         config:
+          disallow_iframe_embed: "True"
           secure_proxy_ssl_header: "True"
           horizon_images_upload_mode: direct
           openstack_enable_password_retrieve: "True"