celery: use Celery 3 config settings instead of deprecated

As warned by:
The 'CELERYD_LOG_LEVEL' setting is scheduled for deprecation in version 2.4 and removal in version v4.0. Use the --loglevel argument instead
remove celeryd.log.file and celeryd.log.level from the ini file.
Instead, use:
paster celeryd my.ini --loglevel=DEBUG --logfile=my.log
or, in the future:
gearbox celeryd -c my.ini -- --loglevel=DEBUG --logfile=my.log

As warned by:
The 'BROKER_VHOST' setting is scheduled for deprecation in version 2.5 and removal in version v4.0. Use the BROKER_URL setting instead
The 'BROKER_HOST' setting is scheduled for deprecation in version 2.5 and removal in version v4.0. Use the BROKER_URL setting instead
The 'BROKER_USER' setting is scheduled for deprecation in version 2.5 and removal in version v4.0. Use the BROKER_URL setting instead
The 'BROKER_PASSWORD' setting is scheduled for deprecation in version 2.5 and removal in version v4.0. Use the BROKER_URL setting instead
The 'BROKER_PORT' setting is scheduled for deprecation in version 2.5 and removal in version v4.0. Use the BROKER_URL setting instead
change the .ini template to use:
broker.url = amqp://rabbitmq:qewqew@localhost:5672/rabbitmqhost

As warned by:
Starting from version 3.2 Celery will refuse to accept pickle by default.

The pickle serializer is a security concern as it may give attackers
the ability to execute any command. It's important to secure
your broker from unauthorized access when using pickle, so we think
that enabling pickle should require a deliberate action and not be
the default choice.

If you depend on pickle then you should set a setting to disable this
warning and to be sure that everything will continue working
when you upgrade to Celery 3.2::

CELERY_ACCEPT_CONTENT = ['pickle', 'json', 'msgpack', 'yaml']

You must only enable the serializers that you will actually use.
change the .ini template to use:
celery.accept.content = pickle

(Note: The warning is there for a reason. It would probably be nice to change
from pickle to something like json. That is left as an exercise.)