Adds configuration for discovery-dhcp

Uses oac to write a dnsmasq.conf - introduces three new env vars:
DISCOVERY_INTERFACE - for the interface onto which the discovery
dnsmasq process will be bound (default br-ctlplane),
DISCOVERY_IPRANGE - for the dhcp address range to be given to
new hardware going through discovery (192.0.2.100.192.0.2.120),
DISCOVERY_PXIP - the IP address for the PXE server (192.0.2.1)