ENH: Allow ExternalData path to be set by environment on dashboards

The new priority for setting the value of "ExternalData_OBJECT_STORES"
that is passed to any new ITK build is:
1. A CMake variable in the client configuration, "ExternalData_OBJECT_STORES"
2. An environment variable, "ExternalData_OBJECT_STORES"
3. The path "${CTEST_DASHBOARD_ROOT}/${dashboard_data_name}"
4. The path "${CTEST_DASHBOARD_ROOT}/ExternalData"

Change-Id: I4ec8dde3d541fda464dc72a7e7f99d5c0b0c1072
Merge "ENH: Allow ExternalData path to be set by environment on dashboards"