Add local_settings.d/ dir for settings snippets

All files ending with '.py' in local_settings.d/ are evaluated
after reading the settings from local_settings.py . That means
changing the settings can be done without changing local_settings.py .

Change-Id: I0b25ef2c4dada23f8cb66aa5e8af44b3a1cba3b9
Implements: blueprint local-settings-override-mechanism

doc/source/topics/settings.rst

@@ -292,6 +292,23 @@ Most of the following settings are defined in
  ``openstack_dashboard/local/local_settings.py``, which should be copied from
  ``openstack_dashboard/local/local_settings.py.example``.
 
+Since Mitaka, there is also a way to drop file snippets into
+``openstack_dashboard/local/local_settings.d/``. These snippets must end with
+``.py`` and must contain valid Python code. The snippets are loaded after
+``local_settings.py`` is evaluated so you are able to override settings from
+``local_settings.py`` without the need to change this file.
+Snippets are evaluated in alphabetical order by file name.
+It's good style to name the files in ``local_settings.d/`` like
+``_ZZ_another_setting.py`` where ``ZZ`` is a number. The file must start with
+an underscore (``_``) because Python can not load files starting with a number.
+So given that you have 3 files, ``local_settings.py``,
+``local_settings.d/_10_setting_one.py`` and ``local_settings.d/_20_settings_two.py``,
+the settings from ``local_settings.py`` are evaluated first. Settings from
+``local_settings.d/_10_settings_one.py`` override settings from ``local_settings.py``
+and settings from ``local_settings.d/_20_settings_two.py`` override all other settings
+because that's the file which is evaluated last.
+
+
 ``AUTHENTICATION_URLS``
 -----------------------
 

openstack_dashboard/local/local_settings.d/_10_set_custom_theme.py.example

@@ -0,0 +1,2 @@
+# override the CUSTOM_THEME_PATH variable with this settings snippet
+# CUSTOM_THEME_PATH="themes/material"

openstack_dashboard/settings.py

@@ -267,6 +267,19 @@ try:
 except ImportError:
     logging.warning("No local_settings file found.")
 
+# allow to drop settings snippets into a local_settings_dir
+LOCAL_SETTINGS_DIR_PATH = os.path.join(ROOT_PATH, "local", "local_settings.d")
+if os.path.exists(LOCAL_SETTINGS_DIR_PATH):
+    for (dirpath, dirnames, filenames) in os.walk(LOCAL_SETTINGS_DIR_PATH):
+        for filename in sorted(filenames):
+            if filename.endswith(".py"):
+                try:
+                    execfile(os.path.join(dirpath, filename))
+                except Exception as e:
+                    logging.exception(
+                        "Can not exec settings snippet %s" % (filename))
+
+
 if not WEBROOT.endswith('/'):
     WEBROOT += '/'
 if LOGIN_URL is None:

releasenotes/notes/bp-local-settings-override-mechanism-6c8632432cf1f44c.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - Allow to override settings from local_settings.py with
+    file snippets dropped into local_settings.d/ directory.