Add unique constraint to InstanceTypeExtraSpecs.

Added unique constraint
'uniq_instance_type_extra_specs0instance_type_id0key0deleted'
('instance_type_id', 'key', 'deleted')
to InstanceTypeExtraSpecs model and migrate
scripts.

Exception from UC violation used in method
flavor_extra_specs_update_or_create()
for race condition check.
Get-update-create procedure will be restarted
in case race condition.

blueprint db-enforce-unique-keys

Change-Id: Ie91c6fc19ca01526b5f48f85cbb63a6842f9c890