Reboot only if configured in environment variable

The changes in src/monk_tf/devicetestcase.py and src/monk_tf/device.py
make the reboot at start and end of a devicetestcase optional. The
devicetestcase is processing no reboot if the environment variable
"MONK_NEED_REBOOT" is set "0". In default the reboot is done also if
there is no environment variable set.