execution support for carrying out MGS-managed upgrades

[davepacheco]

This ticket involves taking the blueprint configuration for an SP that will be added in #7414 and making it reality by performing the SP update during blueprint execution. We expect this process to be very similar for SP, RoT, and host phase 1 image upgrade (the three MGS-managed upgrades).

There are a bunch of details to be worked out here to deal with the usual problems that these upgrade processes are designed around a single, stateful consumer, but we have three Nexus instances. I'll try to compile the various notes we have elsewhere on ideas here.