Add unique constraint to AggregateMetadata

Added unique constraint
'uniq_aggregate_metadata0aggregate_id0key0deleted'
('aggregate_id', 'key', 'deleted') to
AggregateMetadata model and migrate
scripts.

Exception from UC violation used in method
aggregate_metadata_add() for race condition check.

blueprint db-enforce-unique-keys

Change-Id: I793706964ce76340c87e536af35a2dfa37408869
Merge "Add unique constraint to AggregateMetadata"