'scan-for-best-tpr3.rb' is now launched by a10-watchdog

So we don't need to launch a10-watchdog from this script itself
anymore. Also we don't issue a "reboot" command at the end, but
return an exit code for the external a10-watchdog tool. This works
a bit nicer for testing purposes when we don't particularly want
to reboot the system after this script is done.
scan-for-best-tpr3.rb: Try harder to actually set tpr3

Sometimes running the a10-set-tpr3 tool causes an instant
fatal failure (due to it being not really nice and correct)
and the test gets pessimistically declared as failed.

Now we give it a number of tries to attempt to pass beyond
this stage.
scan-for-best-tpr3.rb: Log the current tpr3 value for debugging

Log the information about the current tpr3 value to a new status
file '_current_work.txt' file. This can simplify debugging and
also allows to easier tracking of the progress.