I74b9de7092f182c942a58ac7a46b9fbd791889de hit a common ansible gotcha
where it likes to strip the trailing newline after a {% endif %}.
This has resulted in invalid lines in our sources.list.
Unfortunately we miss this because it still exits with 0. Add a
simple test looking for warning output.
Change-Id: I46d393a5e67d10a52c4dcca803176ff368a4b5bd
c20a2435a6