systemd-conf:wired.network: do not send DHCP release on service stop

Fix the problem with floating IP addresses after first reboot in Edgehog image.

Even when a stable MAC is assigned to device via device-tree, an IP address may change after the first reboot.
This is due to default implementation of built-in DHCP client provided by systemd-network daemon - DHCP RELEASE packet is sent to the DHCP server during reboot by this daemon. So, next time a new IP address is allocated to the same DHCP client (device).

To change this behavior of the DHCP client, a "KeepConfiguration=dhcp-on-stop" option is enabled in configuration file for systemd-networkd.