Rename instance_type to flavor in nova.utils and nova.compute.utils

Renames 'instance_type' to 'flavor' in nova.utils and nova.compute.utils
and related tests. Removes the unused fake_get method in
test_compute_utils.

Note that the nova.notifications payload isn't updated yet to store
flavor instead of instance_type so this patch still uses
payload['instance_type'] when processing the notification in
test_compute_utils.

Note that instance_type_id and the instance_type_* keys in
system_metadata are part of the data model and would require a database
migration to change, so those are not part of this patch.

Part of blueprint flavor-instance-type-dedup

Change-Id: I56815e5c8238f8e4ad4e3a2a599310da2b7f4568
Merge "Rename instance_type to flavor in nova.utils and nova.compute.utils"