OpTestSystem: Exit the test when BMC failed to power off the system
OpTestSystem: Exit the test when BMC failed to power off the system

Currently  sys_power_off returns rc(1) when BMC failed to issue power
off, in state machine no where we are checking for the return code to
exit the test. Hence tests are continuing even the power off request
fails,. This patch removes those return code paths, and make sure
ipmi_power_off raises the exception when BMC failed to power off.

Signed-off-by: Pridhiviraj Paidipeddi <fd8ea14509c256cbd04eaa4ee745a213945e70a1@linux.vnet.ibm.com>

open-power/op-test-framework,open-power/op-test-framework,open-power/op-test-framework