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
Merge "Driver interface's validate should return nothing"