This patch is a oneliner: rangedloop checker should check the subnets only if it's not turned off with placement annotation.
(see in satellite/repair/checker/observer.go).
But I didn't find any unit test to cover that part, so I had to write one, and I prefered to write it as a unit test not an integration test, which requires a mock repair queue (observer_unit_test.go mock.go).
Because it's small change, I also included a small change: creating a elper method to check if AutoExcludeSubnet annotation is defined
Change-Id: I2666b937074ab57f603b356408ef108cd55bd6fd
When we check the availability of the pieces, we do:
```
result.NumUnhealthyRetrievable = len(result.ClumpedPiecesSet) + len(result.OutOfPlacementPiecesSet)
// + some magic if there are overlaps between them
numHealthy := len(pieces) - len(piecesCheck.MissingPiecesSet) - piecesCheck.NumUnhealthyRetrievable
```
This works only if OutOfPlacementPieceSet doesn't contain the offline nodes (which are already included in MissingPieceSet).
But `result.OutOfPlacementPieces.Set` should include all the nodes (even offline), as in case of lucky conditions, we are able to remove those pieces from DB.
The solution is to remove all offline nodes from `NumUnhealthyRetrievable`.
Change-Id: I90baa0396352dd040e1e1516314b3271f8712034
