release-cut: Update process to use unstable's current public snapshots

This alters the release cut scripts to use unstable's snapshots as the basis for
creating the release cut. It also migrates a bunch of functions into the library
so we can better manage the code, and makes the cut idempotent, doing a "publish
switch" instead of a "publish snapshot" in case the releases have already been
published.

Change-Id: Ib4754cb70f6fce37f265b6dfc075025dc69fde99
2 files changed