nm: use device's Disconnect instead of DeactivateConnection

According to NM's documentation:
- connection down: Deactivate a connection from a device without
  preventing the device from further auto-activation.
- device down: Disconnect a device and prevent the device from
  automatically activating further connections without user/manual
  intervention.

The documentation about the D-Bus Device.Disconnect method says the
same.

Use Device.Disconnect to prevent from unwanted connection activations
on devices whose desired state is `down`.

Previously, to avoid uwnanted activations we just deleted any other
connection that matches the device. However, we didn't delete connections
with "multiconnect" that also match other devices to avoid affecting to
other devices. In this case, the multiconnect connection may auto-
activate on the device after doing connection down on the previous
active connection. Using Device.Disconnect it won't happen.

Note that this problem still exists when using `state: absent`. In that
case the multiconnect connection will auto-activate after a reboot. This
needs to be fixed separately.

Signed-off-by: Íñigo Huguet <ihuguet@riseup.net>