Allow piecemeal overrides of tokenserver backend default config.

Before this change, you have either have to accept the default config
for the tokenserver backend, or specify all config options and hence
duplicate info from earlier in the file.  With this change you can
now just specify anything you want to change and we'll use the defaults
for the rest.

syncserver/__init__.py

@@ -22,6 +22,9 @@ from tokenserver.util import _JSONError
 logger = logging.getLogger("syncserver")
 
 
+DEFAULT_TOKENSERVER_BACKEND = "syncserver.staticnode.StaticNodeAssignment"
+
+
 def includeme(config):
     """Install SyncServer application into the given Pyramid configurator."""
     # Set the umask so that files are created with secure permissions.
@@ -53,11 +56,16 @@ def includeme(config):
     settings.pop("config", None)
     if "tokenserver.backend" not in settings:
         # Default to our simple static node-assignment backend
-        settings["tokenserver.backend"] =\
-            "syncserver.staticnode.StaticNodeAssignment"
-        settings["tokenserver.sqluri"] = sqluri
-        settings["tokenserver.node_url"] = public_url
-        settings["endpoints.sync-1.5"] = "{node}/storage/1.5/{uid}"
+        settings["tokenserver.backend"] = DEFAULT_TOKENSERVER_BACKEND
+    if settings["tokenserver.backend"] == DEFAULT_TOKENSERVER_BACKEND:
+        # Provide some additional defaults for the default backend,
+        # unless overridden in the config.
+        if "tokenserver.sqluri" not in settings:
+            settings["tokenserver.sqluri"] = sqluri
+        if "tokenserver.node_url" not in settings:
+            settings["tokenserver.node_url"] = public_url
+        if "endpoints.sync-1.5" not in settings:
+            settings["endpoints.sync-1.5"] = "{node}/storage/1.5/{uid}"
     if "tokenserver.monkey_patch_gevent" not in settings:
         # Default to no gevent monkey-patching
         settings["tokenserver.monkey_patch_gevent"] = False