Rename instance_type parameter in migrate_disk_and_power_off to flavor

Renames the 'instance_type' parameter in the nova virt driver's
migrate_disk_and_power_off method to 'flavor' and handles the immediate
related code changes, i.e. the method itself, the implementations in the
various virt drivers, the tests, and anything called solely from
migrate_disk_and_power_off. This patch does not adjust for everything
that calls into migrate_disk_and_power_off, like from the nova compute
manager code.  That will be handled in a later patch in the series.

Part of blueprint flavor-instance-type-dedup

Change-Id: I47f2b9f6597556dca632d94f83cc1428ad73ae74
Merge "Rename instance_type parameter in migrate_disk_and_power_off to flavor"