Change validate() to raise instead of returning T/F
Change validate() to raise instead of returning T/F

Robert was right - it's better to allow driver.interface.validate to
raise an exception, and let that bubble up and be wrapped by the API, or
logged, than to wrap it at a low level and convert to True/False.

This patch changes the existing drivers' validate methods to remove
the exception wrapping, and updates some doc strings.

Change-Id: I02372db64ea0eb85a3c0a00de292c87d286808e1

citrix-openstack-build/ironic-lib,faizan-barmawer/ironic-lib,faizan-barmawer/elytics,openstack/ironic-lib,NaohiroTamura/ironic-lib