Fix db.models.Service description

DB models definition should be up-to-date with DB schema obtained after applying of
migrations. Currently, there are no tests enforcing this and it's really hard to
analyze what indexes should be added, and what indexes we have at the moment.

So the first step is it to fix all models, and add missing descriptions about
Indexes and UniqueConstraints in __table_args__ and use explicit 'nullable'
parameter in columns description.

Add missing 'nullable' parameters to all Columns

Add empty __table_args__ parameter to Table

There is no indexes and no unique constraints in this table at this moment.

blueprint db-sync-models-with-migrations

Change-Id: I9b31e8976f514e370d6b46076b469fe5e5c8e0f1
Merge "Fix db.models.Service description"