Add 'flavor' to fakes.py

In the tests/fakes.py module, add to the FakeNovaClient class the
attribute flavor. This leads to the addition of the FakeFlavorManager class
(with one method; create) and the FakeFlavor class.

Change-Id: Ia883b249b17ac74cc1fb3bd9c876c2e9dd5d9f14
Merge "Add 'flavor' to fakes.py"