ipmitool's _power_status raises IPMIFailure

Modified ipmitool's _power_status() so that it raises an IPMIFailure exception
instead of a non-IronicException that the ipmitool call might raise. With this
change, ipmitool only returns IronicExceptions.

Also, made minor change to IPMIToolDriverTestCase.test_set_power_invalid_state()
to specifically test for an InvalidParameterValue exception (instead of any
IronicException).

Change-Id: Ieb7dd9fe50737dce6a5c6c6c9142b944a00859d6
Partial-Bug: #1267693
Merge "ipmitool's _power_status raises IPMIFailure"