Add limits for stage_names and environment

[wiktorn]

Add validation to FAST variables. This follows "static" allocation of namespace to specific types, i.e.:

• prefix 9 characters
• environment 4 characters
• stage 6 characters

etc.

It is possible, with more complex code, to check against current variables (i.e., check how long the prefix is, and what's the longest environment name, stage name etc.) and do the validation on the resource_names with checks or pre-condition on null resource.

---

Checklist

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