Power driver for SNMP-enabled smart PDUs

A mechanism for remote control of node power by enabling or disabling
sockets on a rack power strip. The result will be a much wider
generalisation of the hardware that can be controlled by Ironic.

Change-Id: Ib16781eaa9737525c4d8a9231ecd9e72d7e2645e
Merge "Power driver for SNMP-enabled smart PDUs"