Add unique constraints to Quota.

Added unique constraint 'uniq_quotas0project_id0resource0deleted'
('project_id', 'resource', 'deleted') to Quota model and migrate sripts.
Added new exception `QuotaExists`. Updated quota_create(). Change
quotas duplicates control in create/update code to new DB UC exception.
Tests updated respectively.

blueprint db-enforce-unique-keys

Change-Id: Ia615a1c1a7dc75bd19831fcd0acfc9b78c1b3f6f
Merge "Add unique constraints to Quota."