Fix logic for OPENSTACK_HEAT_STACK param

according documentation
https://docs.openstack.org/developer/horizon/topics/settings.html#openstack-heat-stack

Change-Id: I07c84a3a958b02dbe8d92faec21bbfbf71ce59b6
diff --git a/horizon/files/horizon_settings/_heat_settings.py b/horizon/files/horizon_settings/_heat_settings.py
index 7f168c3..acb4a71 100644
--- a/horizon/files/horizon_settings/_heat_settings.py
+++ b/horizon/files/horizon_settings/_heat_settings.py
@@ -6,6 +6,6 @@
 {%- set app = server %}
 {%- endif %}
 
-{%- if app.get('openstack_heat_stack', false) %}
-OPENSTACK_HEAT_STACK = {'enable_user_pass': True}
+{% if app.openstack_heat_stack is defined %}
+OPENSTACK_HEAT_STACK = {'enable_user_pass': {% if app.openstack_heat_stack %}True{% else %}False{% endif %}}
 {%- endif %}