Add voicemail parameter storage-dir-shared

If this is set to true (default false) we assume the storage-dir is
shared, organized by realm/domain, with uses other than voicemail, so
we adopt a directory structure of:

  <storage_dir>/<realm>/voicemail/<id>
Force sofia rebuild

The rebuild is needed for commit d3b9aaba60fa69e84bfad0f55b5abef61225a671.