driver interface' validate() should not return anything

[Ruby Loo]

Some of the driver interface's validate(), eg pxe.VendorPassthru.validate() returns True. The expected behaviour is for them to raise an exception if validation isn't successful, or return nothing if validation succeeds. In addition to fixing the code, we should add something to the base interface docstrings so that it is clear that nothing should be returned.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/97855

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/97855
Committed: https://git.openstack.org/cgit/openstack/ironic/commit/?id=2891f18d598412b8ae48bccd09f331d8013aa20a
Submitter: Jenkins
Branch: master

commit 2891f18d598412b8ae48bccd09f331d8013aa20a
Author: Ruby Loo <email address hidden>
Date: Wed Jun 4 16:47:50 2014 +0000

    Driver interface's validate should return nothing

    Some of the driver interface's validate(), eg pxe.VendorPassthru.validate()
    returns True. The expected behaviour is for them to raise an exception if
    validation isn't successful, or return nothing if validation succeeds.

    This is fixed.

    The docstrings for the validate functions in the base class have been updated
    to mention that they should return None.

    Change-Id: Ic8fb626874ed6cb11494c84dc184d70686441b0f
    Closes-Bug: #1288363