Alter models and add migration

We must have correct models i.e. models that correspond
a 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.

Partial-Bug: #1365436
Change-Id: I9b4571906d39bcdb35048caa58d16ad5e888cce4
Merge "Alter models and add migration"