So, since there is no Terraform module for MongoDB Cloud Manager.
I have created this PoC of MongoDB Cloud Manager API using node.js
It creates a project automatically and replica set with some users already in it and enables monitoring too.
One question for MongoDB, why Digest Authentication, jeez is hard to find a library that supports it!
Please note: Is just PoC, not expect a super awesome code. You can fork it and make it super awesome.