ipmitool does not support OPERATOR priv level

[aeva black]

If the BMC / IPMI credentials being used for management of hardware were only granted OPERATOR privileges, there is no way to inform Nova's baremetal driver or Ironic's ipmitool driver to use this non-default privilege level. These will issue ipmitool commands with no "-L" parameter, resulting in privilege errors, because the default ipmitool privlvl is ADMINISTRATOR.

This could be fixed by adding an optional field to store the privilege level. A null value should indicate default (admin), thus not changing current behaviour.