environment variable for no reboot at start

The devicetestcase.py is changed in order to not do a reboot if it is not
necessary. To configure the reboot at the start of a devicetestcase you
use the environment variable "MONK_NEED_START_REBOOT". Value 1 means
reboot necessary and 0 means reboot not necessary. In default the device
is rebooting at start of the test.
environment variable for no reboot at the end

The device.py is changed in order to not do a reboot if it is not
necessary. To configure the reboot at the end of a devicetestcase you
use the environment variable "MONK_NEED_END_REBOOT". Value 1 means
reboot necessary and 0 means reboot not necessary. In default the device
is rebooting at start of the test.