How to run mongod from the commandline?
Because nothing is written to log file, the first (and only) problem is (probably) ownership of log file. Usermongodb don't have access to that file. sudo chown -R mongodb: /var/log/mongodb should fix that.

What comes to the original question of how to run mongod at the command line and see what is happening, you have two choices. First is comment out (from config file) line what orders mongod to fork itself to background. Second possibility is check from config file,  what parameters you need (like dbPath) and then replace that --config with those command line parameters. You can see list of command line parameters with mongod --help command