set filesystem_store_datadir in scrubber.conf to fix warning

2014-01-27 17:09:18.901 11137 WARNING glance.store.base [ce367618-b8c3-4f2b-b2f7-4138669f2b1a None None] Failed to configure store correctly: Store filesystem could not be configured correctly. Reason: Could not find filesystem_store_datadir in configuration options. Disabling add method.