Make selection of Ethernet connection predictable

Before, the first Ethernet connection in the list given by
`nmcli con show` was selected. However, this was unpredictable and
different connections could be selected on each reboot. Now, the
selected connection is the lexicographically first Ethernet connection.

Another option would be to select the Ethernet connection based on the
physical device name (which could be obtained via the MAC address). See
issue #75. However, this adds needless complication. The user can
manually specify the one they want in their configuration file if the
wrong one is selected by default.
Remove call to `nmcli con down ...` in restart()

Starting with nmcli v0.9.10, `nmcli con down id <con>` fails when the
connection is inactive. Simply calling `nmcli con up id <con>` seems
sufficient to restart the connection anyway, so the `down` call should
be removed.
Changelog entry

Close GH-81, close GH-74, close GH-75.