Improve secrets management

[afeld]

In #1143, we determined that we want to go from the data migration file being treated as one big secret to the file loading secrets it needs at runtime (e.g. as environment variables).

Acceptance Criteria

• We are managing secrets as individual values rather than all in one file
• We minimize the number of places that secrets are stored
• Secrets are stored in a secure location

Tasks

• Modify the existing data migration to read from environment variables - Refactor: add support for env variables in data migration #1198
• Put environment variable values in Key Vault for each environment #1242
  • (support for environment-specific Vaults was added in #830)
• Specify environment variables needed by data migrations as App Service app settings #1241
• Remove LOAD_SAMPLE_DATA option from Benefits migration process #1243
• Delete all data migration files that are no longer needed #1244
  • Delete data migration files from benefits-secrets
  • Delete/rename data migration file from each bucket
  • Delete storage mount from Terraform Azure definition
• Check that documentation accurately reflects new secrets management approach #1245

Additional context

See #1143 and #1116 for more context on past situations that motivated this refactor.

[angela-tran]

Talked with @thekaveman about working on this while we wait for #1090. The first part will be to update our data migration file to read environment variables.

[thekaveman]

Moved description into Milestone, closing.