What network configuration manager am I using?

I recently upgraded 14.04 to 16.04, which includes a transition from Upstart to systemd, and I can't figure out what systemd is doing with my network.

NetworkManager, systemd-networkd, and systemd-resolved are all inactive:

$ systemctl status NetworkManager.service
● NetworkManager.service
   Loaded: not-found (Reason: No such file or directory)
   Active: inactive (dead)
$ systemctl status systemd-networkd
● systemd-networkd.service - Network Service
   Loaded: loaded (/lib/systemd/system/systemd-networkd.service; disabled; vendor pre
   Active: inactive (dead)
     Docs: man:systemd-networkd.service(8)
$ systemctl status systemd-resolved
● systemd-resolved.service - Network Name Resolution
   Loaded: loaded (/lib/systemd/system/systemd-resolved.service; disabled; vendor pre
  Drop-In: /lib/systemd/system/systemd-resolved.service.d
           └─resolvconf.conf
   Active: inactive (dead)
     Docs: man:systemd-resolved.service(8)

No trace of either NetworkManager or systemd-networkd are in my journalctl from the last boot:

$ journalctl -u NetworkManager
-- No entries --
$ journalctl -u systemd-networkd
-- No entries --

Nevertheless, the network is fine. The server is connecting to the DB server and its webapps are internet-accessible as normal. How do I determine what network configuration manager I'm using? I have to be using one, right?

Edit Here are the contents of /etc/network/interfaces:

$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet dhcp
#iface eth0 inet static
#   address x.x.x.x
#   netmask x.x.x.x
#   network x.x.x.x
#   broadcast x.x.x.x
#   gateway x.x.x.x
#   # dns-* options are implemented by the resolvconf package, if installed
#   dns-nameservers x.x.x.x y.y.y.y
#   dns-search x.x.x.x y.y.y.y