More instance_type -> flavor renames in db.api

Renames the instance_type methods in db.api to flavor and converts the
tests to match. Updates doc strings where needed.

Note that this patch doesn't rename anything touching the db models for
instance types, that will be handled in a later patch for the blueprint.

Part of bp flavor-instance-type-dedup

Change-Id: I397421c9b34d295368999c5d085345409945151a
Merge "More instance_type -> flavor renames in db.api"