Rename instance_type to flavor in hyper-v virt driver

Renames 'instance_type' to 'flavor' in the hyper-v virt driver and
associated tests.

Part of blueprint flavor-instance-type-dedup

Change-Id: I769bf86a63283152c5bbc5644e7b74cc2bb24697
Merge "Rename instance_type to flavor in hyper-v virt driver"