MB-100: fix timeouts in rebalanceXDCR suite

Change-Id: I00736a03248e93421f184390f742dda932edc85c
Reviewed-on: http://review.couchbase.org/24836
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>