mongodump shouldn't export config database by default

Currently the mongodump tool exports the config database by default which is in the way of the standard dump -> restore flow. It seems very unlikely people would want to restore a config database by default, if only because it's extremely risky.

Suggest not exporting it by default and add a flag to mongodump to include it if required. Alternatively a parameter to pass a list of databases that shouldn't be exported.