Add a passthrough configuration system

This is an implementation of the strategy 3 from this
comment: netbox-community/netbox#1814 (comment)

Bug: T209182
Change-Id: Icbdf212b7b1f9ff59fef4f5ecf64bfd9fdc41238
(cherry picked from commit 60c58bd)

netbox/netbox/settings.py

@@ -547,6 +547,13 @@ def _setting(name, default=None):
     '::1',
 )
 
+# Settings outside of the scope of the above, additional module settings, etc.
+# CAUTION: This unconditionally sets variables from the passthrough dictionary
+# in the settings module and could lead to odd results.
+passthrough_settings = getattr(configuration, 'PASSTHROUGH', {})
+settings_module = sys.modules[__name__]
+for key, value in passthrough_settings.items():
+    setattr(settings_module, key, value)
 
 #
 # NetBox internal settings