get_dhcp_ha (declared on DhcpServerMixin, optional) is what a DHCP
server's own HA configuration says: on or off, this server's peer name,
each peer's name, role as the server words it, and HA URL.
ConfigSyncMixin.get_config_sync is where a host pushes its configuration
and for which services, by the names get_services reports; sync items
without a service name stay visible under `unmapped`. A driver mixes it
in, no role promises it. Both raise when they could not read.
What the facts mean for a pair -- hot standby, which side to write to --
is the caller's (NetOrk/netork#1125).
christianmanivong