S3BackendPlugin - allow multiple ways to configure

[kmanning]

• S3 Bucket
  • Check for a S3_BUCKET first, if found, use
  • If not found, check for a <ENV>_S3_BUCKET, if found, use
  • if nothing found, do not set backend variable
• Region
  • Check S3_BACKEND_REGION first, if found use,
  • If not found, check for <ENV>_S3_BACKEND_REGION, if found, use
  • If nothing found, do not set backend variable (this should default to AWS_REGION and AWS_DEFAULT_REGION in terraform)
    • See hashicorp/terraform@dd16455
  • Also continue to support DEFAULT_AWS_REGION for backwards compatibility, but deprecate this. This is non-standard.
• Key
  • Allow user to customize
  • If not customized, use the current default terraform/${repoSlug}/${environment}

[kmanning]

This requires a fundamental change to plugin behavior. Prior to the code written for this Issue, once a command was instantiated, new plugins did not take effect.

To enable standard behavior of plugins and the various environment variable hierarchy, we need to delay when plugins are applied to TerraformCommands. In doing so, we inadvertently alter the behavior of plugins - plugins that are added AFTER a TerraformCommand is instantiated still take effect.