CBLT-319:  more robust clean step

Change-Id: Ic47ddf3cb7d6ba6dd5c25ca7e24b3086e16d2e60
Reviewed-on: http://review.couchbase.org/38563
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>