Make possible to use embed iframes

Because horizon local settings cannot use config_template, we
have to maintain the features possible in the template ourselves.

This adds the possibility to allow/deny the use of embedded
iframes.

Change-Id: I12470ac156a433eee795b4090b21d106418ecd5a

releasenotes/notes/horizon_disallow_iframe_embed-8c115e431b812655.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - It's now possible to change the behavior of
+    ``DISALLOW_IFRAME_EMBED`` by defining the variable
+    ``horizon_disallow_iframe_embed`` in the user
+    variables.

templates/horizon_local_settings.py.j2

@@ -725,7 +725,11 @@ REST_API_REQUIRED_SETTINGS = ['OPENSTACK_HYPERVISOR_FEATURES',
 # where iframes are not used in deployment. Default setting is True.
 # For more information see:
 # http://tinyurl.com/anticlickjack
+{% if horizon_disallow_iframe_embed is defined %}
+DISALLOW_IFRAME_EMBED = {{ horizon_disallow_iframe_embed }}
+{% else %}
 #DISALLOW_IFRAME_EMBED = True
+{% endif %}
 
 # The default date range in the Overview panel meters - either <today> minus N
 # days (if the value is integer N), or from the beginning of the current month