ktest: Test if target machine is up before install

Sometimes a test kernel will crash or hang on reboot (this is even more
apparent when testing a config without CGROUPS on a box running
systemd). When this happens, on the next iteration of installing a
kernel, ktest will fail when it tries to install.

Have ktest do a check to see if the target can be connected to via ssh
before it tries to install. If it can't connect, then reboot again.
This time the reboot will fail because it can't connect and will force a
power cycle.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>