Support server power state update through external event

During the regular power sync between ironic and nova, physical instances going down will be marked down in nova's DB. However, physical instances coming up "unexpectedly" (via IPMI or power button) will be shut down again if they are marked as SHUTDOWN in nova. This can cause operational issues, e.g. if there was a power outage or when doing system repairs.

This blueprint proposes to make the power sync "source of truth" configurable, in particular to allow the physical instance to be the source of truth (and make nova update its DB rather than enforcing the DB state onto the physical instance).

The basic idea is to add a "power-update" option/event to the existing os-server-external-events nova API, and then add code to ironic to call that API when a node changes power state and comes back-up.