ENH: Support ExternalData_OBJECT_STORES being set with a Superbuild

Nightly dashboards explicitly set ExternalData_OBJECT_STORES at the top
level Superbuild, but it was previously not being passed down to the
ITKExamples ExternalProject.

Although the ITK ExternalProject's testing is not currently enabled for
Superbuilds, passing down ExternalData_OBJECT_STORES to it will provide
for possible future cases where ITK may require ExternalData even without
testing enabled, or if the ITK ExternalProejct's testing is changed to be
enabled.

This same reasoning applies to passing ExternalData_OBJECT_STORES down
to VTK. Although VTK exposes its own variable, VTK_DATA_STORE, to hold
this value, the ExternalData_OBJECT_STORES variable is still respected by
VTK internally.

Change-Id: I5ed4709d1078715c9ad8229ea097088e22c5f06b
Merge "ENH: Support ExternalData_OBJECT_STORES being set with a Superbuild"