Add a "VTK_DATA_EXCLUDE_FROM_ALL" option to cause the VTKData
to be marked with the EXCLUDE_FROM_ALL property so that it does not
build unless explicitly requested by the user.  This may be useful to
developers that want to get VTK built locally without waiting for
all the test data to download.

Within a dashboard build, hard-code the option to OFF since dashboards
need the data.  Otherwise, select a default based on local
configuration.  Teach the SetupExternalData developer setup script to
prompt for a default.  If that does not provide one, check for an
environment variable.  Otherwise, fall back to a default of OFF so that
the basic build and test sequence works without extra steps.

When the option is ON at the same time as BUILD_TESTING, produce a
warning that a manual step may be needed after building and before
running tests.

Change-Id: Ie35e885c25d9d25bf64653088b97a9c7032b1d0b