Reduce time to publish packages

Eliminate staging mirrors as a step between validating and publishing a package.
At the moment, packages are built and tested in unit and basic deploy tests as part of patch set validation, then pushed to staging mirrors, then staging mirrors are validated using BVT system tests using most recent stable build of Fuel, then staging mirrors are published.
Instead, we must complete all package validation (including BVT) before package change patch set is merged, and then publish packages to the mirrors immediately after review is merged.