Fix project factory blueprint and fast stage

[LucaPrete]

The PR:

• Adds the variable parent to the recently refactored project factory blueprint, so the value can be correctly passed to the project module
• Modifies the project.yaml.sample template file adding the parent key (as opposed to the folder key which seems to map to nothing at the moment)
• Adds the possibility to temporarily set billing id to null in the project_factory blueprint, in order to use project factory internally

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass