Resolve "Create offswitch for testing migration away from .greylist files"

Closes #190 (closed)

This MR adds an offswitch for greylist backwards compatibility. Before merging to master, we'll need to create a CI/CD environment variable in Ironbank Containers with key GREYLIST_BACK_COMPAT and value True.

Successful test jobs:

• Preflight stage, source image doesn't have hardening_manifest.yaml. File is autogenerated. GREYLIST_BACK_COMPAT=True
  • https://repo1.dso.mil/dsop/opensource/pipeline-test-project/ubi8/-/jobs/1441538
• Preflight stage, source image doesn't have hardening_manifest.yaml. GREYLIST_BACK_COMPAT=False
  • https://repo1.dso.mil/dsop/opensource/pipeline-test-project/ubi8/-/jobs/1441437
• Lint stage, base image doesn't have hardening_manifest.yaml. GREYLIST_BACK_COMPAT=True
  • https://repo1.dso.mil/dsop/opensource/pipeline-test-project/grafana/-/jobs/1441855
• Lint stage, base image doesn't have hardening_manifest.yaml. GREYLIST_BACK_COMPAT=False (default)
  • https://repo1.dso.mil/dsop/opensource/pipeline-test-project/grafana/-/jobs/1441871
• Csv-output stage, base image doesn't have hardening_manifest.yaml. GREYLIST_BACK_COMPAT=True
  • https://repo1.dso.mil/dsop/opensource/pipeline-test-project/grafana/-/jobs/1458986
• Csv-output stage, base image doesn't have hardening_manifest.yaml. GREYLIST_BACK_COMPAT=False
  • https://repo1.dso.mil/dsop/opensource/pipeline-test-project/grafana/-/jobs/1458856