Ubuntu – ubuntu server and networkmanager

I have a problem with ubuntu server (16.10) and network manager.

The system is freshly installed (an offline install, with no ethernet cable attached) and at the first boot I plugged the cable into the ethernet port, I simply issued a dhclient; apt-get update; apt-get install network-manager, then I filled a file (/etc/NetworkManager/system-connections/EthernetInterface) with this data:

[802-3-ethernet]
auto-negotiate=true
mac-address=<<ethernet card mac address>>

[connection]
id=EthernetInterface
uuid=<<randomly generated uuid>>
type=802-3-ethernet

[ipv6]
method=ignore

[ipv4]
method=manual
address1=172.16.20.153,172.16.0.254
dns=8.8.8.8;8.8.4.4;

(the ethernet MAC and UUID were properly filled).

Now, this was enough to make the PC automatically connect at boot, but now… No way.

The nmcli d command outputs all the network interfaces as unmanaged. File /etc/network/interfaces contains only the loopback device.

I tried also to edit the NetworkManager.conf file, setting managed=true, but without any luck.

How can I enable the interface? Or how can I track the problem?

Best regards

EDIT:

I know that usually the server it is not managed by NM (in fact it is not even pre-installed).

However I need the NetworkManager features (DBUS interface, automatic fallback on other connections when using mobile interfaces, integration with ModemManager). That's why I installed network-manager. However it is not working: maybe I have to change some default configuration, maybe I have to enable it, or maybe there is some rule somewhere to make it the default network connections handler. Do you know where? Otherwise I'll have to get a desktop version, but I preferred the server version since I didn't need a GUI