WSGI config needs to specify application group

This ensures that the same Python sub-instance handles the request,
even if it's coming in from two different virtual hosts.

docs/wsgi.md

@@ -19,8 +19,9 @@ Then, set up Apache with a configuration like:
 
     <VirtualHost>
         WSGIScriptAlias /nilmdb /home/nilm/nilmdb.wsgi
-        WSGIProcessGroup nilmdb-server
-        WSGIDaemonProcess nilmdb-server threads=32 user=nilm group=nilm
+        WSGIApplicationGroup nilmdb-appgroup
+        WSGIProcessGroup nilmdb-procgroup
+        WSGIDaemonProcess nilmdb-procgroup threads=32 user=nilm group=nilm
 
         # Access control example:
         <Location /nilmdb>