Rename instance_type to flavor in test_utils and nova.tests.utils

Renames occurrences of 'instance_type' to 'flavor' in
nova.tests.test_utils and nova.tests.utils along with any impacted tests
using the changed parts.

Part of blueprint flavor-instance-type-dedup

Change-Id: Ie816356c176c2be61b4a346a4d47d8f3461ae84a
Merge "Rename instance_type to flavor in test_utils and nova.tests.utils"