core: fix check on master active-connection failure

The previous check was never satisfied, as the device is assigned to
the active-connection and realized early. Instead, check the
master_ready flag, which tells if the master is ready and, therefore,
if the slave has already been added to the master.

Before this commit, in some cases a device didn't detect that the
master failed and kept waiting forever.

https://bugzilla.redhat.com/show_bug.cgi?id=1845018
!568
21 jobs for bg/autoconnect-slaves-pt2-rh1845018 in 31 minutes and 42 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #3658187
checkpatch

00:01:10

manual #3658194
allowed to fail manual
t_centos:7.5.1804
manual #3658195
allowed to fail manual
t_centos:7.6.1810
manual #3658196
allowed to fail manual
t_centos:7.7.1908
manual #3658197
allowed to fail manual
t_centos:7.8.2003
manual #3658198
allowed to fail manual
t_centos:8.1.1911
manual #3658204
allowed to fail manual
t_debian:10
manual #3658203
allowed to fail manual
t_debian:9
manual #3658206
allowed to fail manual
t_debian:sid
manual #3658205
allowed to fail manual
t_debian:testing
manual #3658188
allowed to fail manual
t_fedora:28
manual #3658189
allowed to fail manual
t_fedora:29
manual #3658190
allowed to fail manual
t_fedora:30
passed #3658191
t_fedora:31

00:31:42

manual #3658192
allowed to fail manual
t_fedora:32
manual #3658193
allowed to fail manual
t_fedora:rawhide
manual #3658199
allowed to fail manual
t_ubuntu:16.04
manual #3658200
allowed to fail manual
t_ubuntu:18.04
manual #3658202
allowed to fail manual
t_ubuntu:devel
manual #3658201
allowed to fail manual
t_ubuntu:rolling
 
  External
canceled https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/1670/

124:00:55