CBQE-345: add rebalance test with bucket password change

Change-Id: I27993ec53059a59d12e872f9b1e402bae694143a
Reviewed-on: http://review.couchbase.org/24447
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>