Add a custom iboot power driver for nova bm.

Adds a custom power driver for nova baremetal so that
iBoot capable devices can be used.

NOTE: The current implementation only works when there is a single
PDU per baremetal node. In order to support multiple baremetal nodes
attached to a single PDU we'd need to add another pm_relay field in the
database (or something similar).

Tested locally with an iBoot G2.

Change-Id: I155f364f4ff6d77b34ded46de60c5cbca26596a2