Move Fake objects to a special tests.fakes module

A set of Fake objects originally created for nova scenarios unit tests
turned out to be very useful for several other unit tests as well. Here,
we move all these Fake objects to a special tests.fakes module. This will
also simplify code reuse in future unit tests.

Blueprint unit-tests-refactoring

Change-Id: I2b3c6a3f1543a186fe498d886e3fd4bfe615dce8