ktest: introduce REBOOT_RETURN_CODE to confirm the result of REBOOT

ktest: introduce REBOOT_RETURN_CODE to confirm the result of REBOOT

Unexpected power cycle occurs while the installation of the
kernel.

   ssh root@Test sync ... [0 seconds] SUCCESS
   ssh root@Test reboot ... [1 second] FAILED!
   virsh destroy Test; sleep 5; virsh start Test ... [6 seconds] SUCCESS

That is because REBOOT, the default is "ssh $SSH_USER@$MACHINE
reboot", exits as 255 even if the reboot is successfully done,
like as:

   ]# ssh root@Test reboot
   Connection to Test closed by remote host.
   ]# echo $?
   255
   ]#

To avoid the unexpected power cycle, introduce a new parameter,
REBOOT_RETURN_CODE to judge whether REBOOT is successfully done
or not.

Link: http://lkml.kernel.org/r/20190418135943.12640-1-msys.mizuma@gmail.com
Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>