Add unique constraints to AggregateHost.

Added unique constraint 'uniq_aggregate_hosts0host0aggregate_id0deleted'
('host', 'aggregate_id', 'deleted') to AggregateHost model and migrate
scripts.

The implementation of aggregate_host_add() in sqlalchemy.api rewritten.

blueprint db-enforce-unique-keys

Change-Id: If4a6c437d5d4984a6d7ebddfd0924f211533851f