Partial implementation of Driver API.

src/device/service/driver_api/DriverFactory.py

src/device/service/driver_api/Exceptions.py

src/device/service/driver_api/QueryFields.py

src/device/service/driver_api/_Driver.py

src/device/service/driver_api/__init__.py

src/device/service/drivers/__init__.py

src/device/service/drivers/emulated/QueryFields.py

src/device/service/drivers/emulated/__init__.py

src/device/service/drivers/openconfig/__init__.py