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: Ifb2b1bf07c114efb21b18c8fca615f7589ef5715