Add missing tests for db.instance_type_access_* methods

There was no tests in test_db_api for instance_type_access_* methods.

Add tests to ensure that all works.
Also these tests allow us to refactor these methods.
For example add missing unique constraint on columns
(instance_type_id, project_id)

blueprint db-api-tests

Change-Id: Idfeda1685ef8c812c1211783aede15795b0d7caf
Merge "Add missing tests for db.instance_type_access_* methods"