MB-7508: modify test_failed_swap_rebalance

parametrize tests with rebalance percentage when we have to kill memcached and remove loop
according to comments in MB-7508
Rebalance exited with reason {{bulk_set_vbucket_state_failed after crasher in new_ns_replicas_builder(rebalance continued after killing memcached)

Change-Id: I0b2e70c46bbca1844427e8aed20122dc9e3d5b85
Reviewed-on: http://review.couchbase.org/23987
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>