Add PasswordAuthentication=no to first ssh command in wait_for_ssh

It now newly sporadically happens that GCE instance just after
creation requires SSH password authentication. We don't support
this scenario and the password prompt blocks our terminal so the
whole test dies. This is disabling the password authentication
leaving us only key authentication which should be always available.

lib/publiccloud/instance.pm

@@ -414,7 +414,8 @@ sub wait_for_ssh {
         script_run("ssh-keyscan $args{public_ip} | tee -a ~/.ssh/known_hosts");
         while (($duration = time() - $start_time) < $args{timeout}) {
             # timeout recalculated removing consumed time until now
-            $sysout = $self->ssh_script_output(cmd => 'sudo systemctl is-system-running',
+            # We don't support password authentication so it would just block the terminal
+            $sysout = $self->ssh_script_output(cmd => 'sudo systemctl is-system-running', ssh_opts => '-o PasswordAuthentication=no',
                 timeout => $args{timeout} - $duration, proceed_on_failure => 1, username => $args{username});
             # result check
             if ($sysout =~ m/initializing|starting/) {    # still starting