Added a config variable for database name called MONGODB_DB_NAME

In operation_primatives.py there was an assumption that the name of the mongo database was the same as the value of the APP_NAME environmental variable. I added the option of explicitly specifying the name of the database. The scenario I wished to enable was the one where multiple applications share a single database, which is particularly desirable when you are using a hosted MongoDB offering that is charged for by the database. This new environmental variable is called MONGODB_DB_NAME. If it's not specified then APP_NAME will be used.