ENH: Add support for conditional upload to extension driver script

By default enabled, this option will allow to disable the upload of
extension while testing if they can be built.

This will be particularly useful when setting VTK6 dashboards for
the extensions.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23179 3bd1e089-480b-0410-8dfb-8563597acbee