PDU/IPMI: Be less aggressive with IPMI commands

PDU/IPMI: Be less aggressive with IPMI commands

The machine rimava0 doesn't power off or power on when we keep doing
"power status" then "power off" ipmi command without waiting.

Currently, osstest does:
    loop
	ipmitool power status
	ipmitool power off
	sleep 1

This loop goes on forever, the machine never change power state.
Adding some wait in between both command works.

While here, test for timeout just after a status check instead of
sending the `power off` command again which we would never check the
result of.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Ian Jackson <ijackson@chiark.greenend.org.uk>