[#2977] Don't remove ckan/config/celery-supervisor.conf after all.

A previous commit for #2977 removed `ckan/config/celery-supervisor.conf`
as part of deprecating the old Celery background task system. However,
the old documentation told people to copy *or link* that file, so
removing it could break existing installations. Hence this commit
restores the file, it should be kept around until support for the Celery
system is removed completely.

ckan/config/celery-supervisor.conf

@@ -0,0 +1,31 @@
+; ===============================
+; ckan celeryd supervisor example
+; ===============================
+
+; symlink or copy this file to /etc/supervisr/conf.d 
+; change the path/to/virtualenv below to the virtualenv ckan is in.
+
+
+[program:celery]
+; Full Path to executable, should be path to virtural environment,
+; Full path to config file too.
+
+command=/path/to/pyenv/bin/paster --plugin=ckan celeryd --config=/path/to/config/testing.ini
+
+; user that owns virtual environment.
+user=ckan
+
+numprocs=1
+stdout_logfile=/var/log/celeryd.log
+stderr_logfile=/var/log/celeryd.log
+autostart=true
+autorestart=true
+startsecs=10
+
+; Need to wait for currently executing tasks to finish at shutdown.
+; Increase this if you have very long running tasks.
+stopwaitsecs = 600
+
+; if rabbitmq is supervised, set its priority higher
+; so it starts first
+priority=998