Change db.api.instance_type_ to db.api.flavor_

Rename nova.db.api and nova.db.sqlalchemy.api functions from
instance_type_ to flavor_.  Since this is a fairly large change, leave
backwards compatibility in until all dependant changes (moving code from
db.instance_type_*).

Partially implements bp flavor-instance-type-dedup

Change-Id: Ie81c66e13056537ca26ff04defdb6eb86d5c462a
Merge "Change db.api.instance_type_ to db.api.flavor_"