Standardize use of nova.db

We already mostly use:
from nova import db
instead of:
import nova.db.api

So change remaining few cases to use nova.db instead of nova.db.api.
Help make code more uniform and easy to work with.

Change-Id: I6183352c4bbe77b88a5b277788f588b0b48c6476
Merge "Standardize use of nova.db"