L2SS-680: Add empty PDU device

.gitlab-ci.yml

CDB/LOFAR_ConfigDb.json

docker-compose/device-pdu.yml

...hon-profiles/stationcontrol-jupyter/startup/01-devices.py

sbin/run_integration_test.sh

tangostationcontrol/docs/source/devices/pdu.rst

tangostationcontrol/docs/source/index.rst

tangostationcontrol/setup.cfg

tangostationcontrol/tangostationcontrol/devices/README.md

tangostationcontrol/tangostationcontrol/devices/boot.py

tangostationcontrol/tangostationcontrol/devices/pdu.py

...ntrol/integration_test/default/devices/test_device_pdu.py