Provide configuration for upcoming dandischema release with vendorization support

[yarikoptic]

In

• TEMP: sketch on api.tf on what common variables to vendorize and where to pass #222

I tried to remove duplication and generalize more but likely some syntax is not even supported by terraform. But we need to precede the merge (and later release) of

• Vendor-Configurable Metadata Models dandi-schema#294

with addition of terraform definitions for the production and staging instances so they are in effect when new dandischema is released!

May be @aaronkanzer could help as well

[asmacdo]

@aaronkanzer maybe you can help me clarify some things before I can work with this:

• terraform cloud credentials
• terraform cloud walkthrough (ie how to run, how to test)
• is it possible to run locally with --dry-run or something to clean up before testing on staging?

[yarikoptic]

@candleindark Please prepare pr so we can see/discuss if needed against specific changes