Add custom output path for the DB migrate lambda

[tgvashworth]

We had a problem where we were seeing Terraform config drift and unstable plans because the db_migrate_lambda.zip file was being written inside the terraform module folder, which can be outside the repo folder (e.g. if TF_DATA_DIR is set). That means there is no zip file (or index.py) available to a fresh plan and so TF detects a deletion.

A solution to this is to commit or to cache the zip file across builds, but this TF module does not allow that location to be customised.

This PR makes two changes:

1. The contents of the migrate lambda Python file are now inlined, which makes for one fewer files to manage
2. There's a variable that specifies the output location so it can be written to the repo/cache location

To test this:

• I linked the repo that is using this module to our remote branch and verified that the plan became stable and nothing else changes
• I checked the contents of the zip file to verify it still contained a single index.py file
• I did not verify the lambda behaviour (not sure how; happy to try)

[tgvashworth]

@oavdeev Hey! Do you think someone will be able to have a look at this soon?

[tgvashworth]

Following up on this: we are using a fork of the TF config with this commit, and the lambda etc is all working fine.

[leogargu]

This would fix the perpetual diff we see in our pipelines! ✨ Can it be merged please? Happy to help if needed.

[oavdeev]

Merged as #81