Handle IPMI transient failures better.

Sometimes, IPMI "power status" doesn't return either "on" or "off".
When this happens, the IPMI driver shouldn't assume what the state is,
and the baremetal driver shouldn't assume that NOSTATE means SHUTDOWN.

This patch also improves the doc strings for the IPMI driver.

Fix bug 1178378.

Change-Id: I54a5e6309f68ab8e5601e65039366d7d87c03478
Merge "Handle IPMI transient failures better."