manager: propagate the for-user-request flag for slaves autoconnection

If the master is activated by user, propagate the for-user-request to
slaves activations when autoconnecting slaves, so that they can manage
slaves device as needed.

Reproducer:

 ip l add eth1 type veth peer name eth2
 ip l set eth1 up
 ip l set eth2 up
 sleep 2

 echo " * Initial state"
 echo " - eth1: $(nmcli -g general.state device show eth1)"

 nmcli con add type ethernet ifname eth1 con-name slave-test+ master br-test slave-type bridge
 nmcli con add type bridge ifname br-test con-name br-test+ connection.autoconnect-slaves yes ip4 172.25.1.1/24

 nmcli con up br-test+

 echo " * After user activation"
 echo " - br-test: $(nmcli -g general.state device show br-test)"
 echo " - eth1: $(nmcli -g general.state device show eth1)"

should give:

 * Initial state
 - eth1: 10 (unmanaged)
 * After user activation
 - br-test: 100 (connected)
 - eth1: 100 (connected)
14 jobs for bg/slaves-autoconnect-managed in 33 minutes and 1 second (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #401722
allowed to fail
checkpatch

00:01:12

manual #401727
allowed to fail manual
t_centos:7.5.1804
passed #401728
t_centos:7.6.1810

00:24:11

passed #401731
t_debian:9

00:18:05

passed #401733
t_debian:sid

00:12:11

manual #401732
allowed to fail manual
t_debian:testing
passed #401723
t_fedora:28

00:26:15

passed #401724
t_fedora:29

00:32:59

passed #401725
t_fedora:30

00:27:02

manual #401726
allowed to fail manual
t_fedora:rawhide
passed #401729
t_ubuntu:16.04

00:11:46

passed #401730
t_ubuntu:18.04

00:19:09

 
  External
passed NetworkManager test verification

passed NetworkManager test verification

05:41:07