Sync models for AgentBuild, Aggregates, AggregateHost tables.

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 empty __table_args__ parameter to Table

blueprint db-sync-models-with-migrations

Change-Id: I76e064cf11d8b0df473c6085271b48a81fe31702
Merge "Sync models for AgentBuild, Aggregates, AggregateHost tables."