PDU::ipmi_try: New method

PDU::ipmi_try: New method

For power on, this is just like ipmi.

For power off, it doesn't mind errors (although it has three
attempts).  This is useful when combined with pdu power: if the
chassis power is already off, ipmi will fail.

If the ipmi operation does fail, then the chassing pdu power operation
will do the job.  So this will also fix things if the test case
crashed the bmc or something.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>