cli: use nm_client_add_connection2() API from nmcli

Make use of the new API. Note that AddConnection2() covers all
functionality of AddConnection() and AddConnectionUnsaved(). Let's
only use one API.

There is a minor downside to this patch: now nmcli requires
libnm 1.20 API. Note that libnm's nm_client_add_connection2()
makes an effort to avoid AddConnection2() under the hood to
still work against older server versions. nmcli here does not
do that w.r.t. requiring libnm 1.20. I think that is sensible
because commonly nmcli requires a libnm version that is as new
as itself. Also, usually the same package provides nmcli and libnm.
20 jobs for th/settings-add2-and-update2-options in 125 minutes and 53 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Test
passed #431072
allowed to fail
checkpatch

00:01:09

manual #431077
allowed to fail manual
t_centos:7.5.1804
passed #431078
t_centos:7.6.1810

00:28:22

passed #432584
t_debian:10

00:13:25

passed #432585
t_debian:9

00:33:07

passed #432583
t_debian:sid

00:34:42

manual #431085
allowed to fail manual
t_debian:testing
passed #431073
t_fedora:28

00:45:57

passed #431074
t_fedora:29

00:36:40

passed #431075
t_fedora:30

00:23:37

manual #431076
allowed to fail manual
t_fedora:rawhide
passed #431079
t_ubuntu:16.04

00:36:55

passed #431080
t_ubuntu:18.04

00:25:26

manual #431082
allowed to fail manual
t_ubuntu:devel
manual #431081
allowed to fail manual
t_ubuntu:rolling
failed #431084
t_debian:10

00:00:12

failed #431083
t_debian:9

00:18:59

failed #431086
t_debian:sid

00:00:12

 
  External
passed NetworkManager test verification

passed NetworkManager test verification

05:58:17