Fix power manager hangs while executing ipmitool

In nova/virt/baremetal, IPMI power manager uses ipmitool with '-F' option to
specify a file containing the remote server password.  When the passowrd was
empty, ipmitool ignores that file and asks user to set password in stdin.
This makes the thread stop.

This patch changes power manager to write '\0' to the password file in the
case of empty password, to force the password on impitool.

Closes-bug: #1237802

Change-Id: I44f56129a8ce2af4dda8dea35eb9c60d9518d54b
Merge "Fix power manager hangs while executing ipmitool"