Alter models and add migration

We must have correct models i.e. models that correspond
database schema to use sqlalchemy features.
Update models.py and add migration script to correct
database schema and get rid of the difference between
schema and models.
Remove Stack.user_creds_id nullable=False parameter
because  we can soft-delete stack, that leads to deletion
user_credits and setting stack.user_creds_id to null
(by cascade).
Remove resource.id default value(in database) as specified
in models.
Since we will have sync check between models and migrations
we need to add missing indexes to models for consistency.

Partially implements bp: db-sync-models-with-migrations

Change-Id: Iabc4aced0a95e29cea9c48abc4c31be98e8ff9b1
Merge "Alter models and add migration"