Use the new "sexy" validations for SnapshotDiffCluster

As pointed out by Rubocop. This resolves all of the cops for this file.

Change-Id: I51fa7444f9f717f3f2848ff1bb87bd409ebbbe7b
Use the new "sexy" validations for SnapshotDiff

As pointed out by Rubocop. This resolves all of the cops for this file.

Change-Id: Id82cf7a6ba34c6de958ed3064ceae683a14f5fdf