Added a config variable for database name
Small change to how MONGODB_DB_NAME is checked

Before the code would try to get the database reference using APP_NAME and then try to get it again using MONGODB_DB_NAME if it was set.  While this worked it didn't match the intent.  Now the proper name is evaluated first before the code attempt to get a reference.
small change
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 which is necessary if it's different than APP_NAME. This new environmental variable is called MONGODB_DB_NAME. If it's not specified than APP_NAME will be used.
Merge branch 'master' of https://github.com/FityFity/lda-serverlib