Handle any MongoDB connection option inline

Currently there is a mix of connection options for mongodb are confusing as there is a nested options dictionary for extra options.

    CELERY_MONGODB_BACKEND_SETTINGS={
        "host": "localhost",
        "port": 27017,
        "database": "celery_backend",
        "options": {"use_greenlets": True},  # use_greenlets has to be nested
        "taskmeta_collection": "my_taskmeta_collection"
    }

This patch allows you to state options inline eg:

    CELERY_MONGODB_BACKEND_SETTINGS={
        "host": "localhost",
        "port": 27017,
        "database": "celery_backend",
        "use_greenlets": True,
        "taskmeta_collection": "my_taskmeta_collection"
    }

Also sets `use_greenlets` automatically if using gevent.