Fix db.models.ComputeNode 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 in ComputeNode model definition.

blueprint db-sync-models-with-migrations

Change-Id: I13c92bdd33eede97bb7b2fc45cbedca556992060
Merge "Fix db.models.ComputeNode description"