Fix db.models.Instance 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

Add 6 missing indexes in Instance model definition.

blueprint db-sync-models-with-migrations

Change-Id: I01025789f00dcf058754ee1def726e3b48ec7fda
Merge "Fix db.models.Instance description"