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
