Make celery LWRP honor virtualenv and CELERY_CONFIG_MODULE

The virtualenv is necessary to make sure that we are enabling the proper version of celery. 

Additionally, the way the non-django path of the code was working would not properly load the CELERY_CONFIG_MODULE due to the fact that celery expects a python module name, but was getting a filename (e.g. celery_settings vs. celery_settings.py).