ipmitool SHOULD accept empty username/password

It is valid (if ill-advised) to supply an empty username or an empty
password to IPMI, however the ipmitool driver does not allow.

Change-Id: I22139f082da693f8991a4cd5ad5955e84888948c
Closes-Bug: #1246342
Merge "ipmitool SHOULD accept empty username/password"