what is the password to user mongodb when mongodb is installed in ubuntu?
Launch Shell as User:

If all else fails, on Ubuntu 14.04 LTS running on AWS, try this...

me@local $ ssh -i aws.pem ubuntu@foo
ubuntu@foo $ sudo -u mongodb bash   # OR whatever shell (zsh)
mongodb@foo $ mongod --dbpath /data/db --fork


OF COURSE, you'll add a much larger set of flags to the command. Note also that this assumes you're authenticated as ubuntu on an AWS EC2 instance. The first portion of each line to the left of the $ is the user@hostname 

Invoke Command as User:

YMMV but this should work too...

mongodb@foo $ sudo -u mongodb mongod --dbpath /data/db --fork