Fixes #20912 - Listen on interfaces in EL7
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Networking_Guide/sec-dhcp-configuring-server.html#starting-stopping-serversuggests to override the full file but this just uses a drop-in tochange as little as possible.
Fixes #20912 - Listen on interfaces in EL7
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Networking_Guide/sec-dhcp-configuring-server.html#starting-stopping-server
suggests to override the full file but this just uses a drop-in to
change as little as possible.