Rename virtapi.instance_type_get to flavor_get

Renames the virtapi.instance_type_get method to flavor_get. Updates the
directly related tests and subclass implementations of VirtAPI.

This patch does not propagate to all related code like conductor API,
that will be handled in a later patch for the blueprint.

Part of bp flavor-instance-type-dedup

Change-Id: Ic47da9aa578d449a4ce542d2d017840f1a8132a7
Merge "Rename virtapi.instance_type_get to flavor_get"