fix ipmitool power.on precheck.

fix below.

- When occured precheck(power.status) error(*), power.on return
  incorrect result value(i.e. true).
  (*) Auth mismatch, IP addr typo, etc.
Merge pull request #20 from kiyohara/fix_ipmitool_pow_on_precheck

fix ipmitool power.on precheck.