[lbussell]

Summary of changes:

• Use separate build and product versions for pre-release images. Previously, we would set both build-version and product-version (in manifest.versions.json) to the product version for stable builds. But for pre-release builds, we need to set the build version explicitly or else the Dockerfile templates have no way to construct the correct URL. This is because in the staging storage account, stable versions of .NET still have the full build version in their blob container name. This makes sense in case multiple versions of the same stable version need to be staged.
• Go back to using ReleaseConfig instead of BuildManifest to resolve product versions. Reasoning:
  • We are no longer concerned with reading *.sha512 files which were listed in the BuildManifest
  • It's easier to read the build vs. product versions directly from ReleaseConfig. BuildManifest lists all the artifacts that were published, but to get the build version we would still have to parse it out of a storage account URL or download and read a second artifact. We can just get everything from the ReleaseConfig instead.
• Only get pre-release build versions from pipeline artifacts, not the storage account. I know that code is relatively new, but the pipeline method is just simpler for now. Now that we use online shas, I think this decision is less impactful. We can also always add the storage account code back from version control if we need it.