Fix db.models.ComputeNodeStats 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 ComputeNodeStats model definition.

blueprint db-sync-models-with-migrations

Change-Id: I10761a6c1b32a457c855aff9603bf2edec44de17
Merge "Fix db.models.ComputeNodeStats description"