lava_helper: Adjusted power-off timeout

* Set the power-off timeout to 50 seconds
* Fixed a string formatting bug in argument-parser
  documentation

Change-Id: I18fd8b20edefe76818ae571f686ddb2cfad42871
Signed-off-by: Minos Galanakis <minos.galanakis@linaro.org>
2 files changed