Add dhcp_server fact.

Add a dynamic fact to interfaces detailing the DHCP server servicing
that interface if any.

Creates: dhcp_server_<iface>

Tries to use `nmcli -d list iface <iface>` if nmcli is available and the
interface is managed using NetworkManager.

If not, it will try hard to find a leases file for the interface and
read the dhcp server from that.