Improve db_sqlalchemy_api test coverage

Added new tests and improved coverage for methods:

 instance_group_*
 migration_*
 task_log_*
 agent_build_*

Blueprint: db-api-tests

Change-Id: Idbf704b3e6aa8df1d4f927e67d108e6a8489b9d2
Merge "Improve db_sqlalchemy_api test coverage"