nova/db/__init__.py was importing * from nova.db.api. This meant that
any time any code anywhere within the nova.db package was imported
then nova.db.api was too, leading to a cascade of imports that may
not have been desired. Also, in general, code in __init__.py is a pain.
Therefore, this change adjusts code that so that either:
* nova.db.api is used directly
* nova.db.api is imported as 'db'
In either case, the functionality remains the same.
The primary goal of this change was to make it possible to import the
model files without having to import the db api. Moving the model files
to a different place in the directory hierarchy was considered, but
given that "code in __init__.py is a pain" this mode was chosen.
This looks like a very large change, but it is essentially adjusting
package names, many in mocks.
Change-Id: Ic1fd7c87ceda05eeb96735da2a415ef37060bb1a