[WFLY-2551] Defer reading DS's override registration until Stage.RUNTIME as it's not added by the add handler until then.

Really the add handler should be adding it in MODEL, but trying that leads to test failures and I don't feel like spending the time to sort them now.
Merge pull request #6563 from bstansberry/WFLY-2551_2

[WFLY-2551] Defer reading DS's override registration until Stage.RUNTIME...