Allow adding to INSTALLED_APPS from local_settings.py

Defines a new option, ADD_INSTALLED_APPS, which gets prepended
to the beginning of INSTALLED_APPS.

Change-Id: I956835c3e18edee9e223a607005b47a5c5977687
Closes-bug: #1343342

doc/source/topics/settings.rst

@@ -977,6 +977,15 @@ are generally safe to use.
 When CSRF_COOKIE_SECURE or SESSION_COOKIE_SECURE are set to True, these attributes
 help protect the session cookies from cross-site scripting.
 
+``ADD_INSTALLED_APPS``
+----------------------
+
+.. versionadded:: 2015.1(Kilo)
+
+A list of Django applications to be prepended to the ``INSTALLED_APPS``
+setting. Allows extending the list of installed applications without having
+to override it completely.
+
 
 .. _pluggable-settings-label:
 

openstack_dashboard/settings.py

@@ -251,6 +251,7 @@ SECURITY_GROUP_RULES = {
     },
 }
 
+ADD_INSTALLED_APPS = []
 try:
     from local.local_settings import *  # noqa
 except ImportError:
@@ -266,6 +267,7 @@ settings.update_dashboards([
     openstack_dashboard.enabled,
     openstack_dashboard.local.enabled,
 ], HORIZON_CONFIG, INSTALLED_APPS)
+INSTALLED_APPS[0:0] = ADD_INSTALLED_APPS
 
 # Ensure that we always have a SECRET_KEY set, even when no local_settings.py
 # file is present. See local_settings.py.example for full documentation on the

openstack_dashboard/utils/settings.py

@@ -138,4 +138,4 @@ def update_dashboards(modules, horizon_config, installed_apps):
     horizon_config.setdefault('angular_modules', []).extend(angular_modules)
     horizon_config.setdefault('js_files', []).extend(js_files)
     horizon_config.setdefault('js_spec_files', []).extend(js_spec_files)
-    installed_apps[:] = apps + installed_apps
+    installed_apps[0:0] = apps