CBQE-0 set priorities for flush conf

Change-Id: I6b0519ee66eb4069747e7804d4c5080b2739ddd9
Reviewed-on: http://review.couchbase.org/25787
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>