CBQE-0: sleep 10 sec after rebalance failed

in _common_test_body_failed_swap_rebalance

Change-Id: I82603978510138ecee069845fc566841792abbc3
Reviewed-on: http://review.couchbase.org/25751
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>