Adds SNMP power driver

Add a new PowerDriver module for remote control of node power by
enabling or disabling sockets on a rack power strip via SNMP.

Implements: blueprint ironic-snmp-power-driver
Change-Id: Ib16781eaa9737525c4d8a9231ecd9e72d7e2645e
Co-authored-by: David Hewson <dhewson@cray.com>
Co-authored-by: Mark Goddard <mgoddard@cray.com>
Merge "Adds SNMP power driver"