CBD-1425:  also capture log of step fetch-manifest.rb

Change-Id: I019ed4cd93df2d85aad7b370fc1b4f15cdc4a4e7
Reviewed-on: http://review.couchbase.org/40512
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>