bug in commissioning-user-data means power_settings always passed from commissioning

[Scott Moser]

/etc/maas/commissiong-user-data (and trunk src/metadataserver/commissioning/user_data.template) say:

   power_settings=$(maas-ipmi-autodetect --configdir "$IPMI_CONFIG_D" ${pargs})
   if [ ! -z "power_settings" ]; then
      signal "--power-type=ipmi" "--power-parameters=${power_settings}" WORKING "finished [maas-ipmi-autodetect]"
   fi

'power_settings' will never be empty, meaning the signal will always run.

the check for '-z' should be in '$power_settings'.