Fix db.models.Certificate 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 2 missing indexes in Certificate model definition.

blueprint db-sync-models-with-migrations

Change-Id: Ida607c2b8899e145116d46cffdc49b7176b05550
Merge "Fix db.models.Certificate description"