Add fake_instance.py

There are lots of places in the tests where we create a piece of
an instance-like dict and return that from a stubbed database
function. This "works" right now, but won't in the future of stricter
objects. This adds a fake_instance.py (to mirror the other ones
already there) which builds an instance database primitive that
includes sane values for every field it should have. This will make
testing much easier going forward with instance objects.

Related to blueprint unified-object-model

Change-Id: I13d97cc101c1964aa91ce48093507bbc88803cbb
Merge "Add fake_instance.py"