getChildFilesystems should include fallback fs as well

Currently getChildSystems API used by many other APIs, like 

getAdditionalTokenIssuers, getTrashRoots etc.

If fallBack filesystem not included in child filesystems, Application like YARN who uses getAdditionalTokenIssuers, would not get delegation tokens for fallback fs. This would be a critical bug for secure clusters.

Similarly, trashRoots. when applications tried to use trashRoots, it will not considers trash folders from fallback. So, it will leak from cleanup logics.