Fix DuplicateProperty handling of nested rule sets

The linter would not report duplicate properties in nested rule sets.

Fix #295 by `yield`ing to continue visiting children.

While here, also did a bit of cleanup.

Change-Id: I73ad40e3cf65d72b0792e77a62e33fb75cc77ac8
Reviewed-on: http://gerrit.causes.com/44730
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>