Add 'anon' kwarg to FakeDbBlockDeviceDict class

This will allow us to create fake block devices without the database id
field, so we can pass them to BDM objects in our test  which can then
call create(). create() would normally error out if there is a DB id set
on the object.

Change-Id: I2d1f975633c8b2cb9e2e744c5f769b8f753b5968
Merge "Add 'anon' kwarg to FakeDbBlockDeviceDict class"