CBQE-1053: fix replication_while_rebooting_a_non_master_destination_node

better to wait when warmup completed instead of hardcode sleep

Change-Id: I6b431d6c892fc779f4330723e5324b4a8a5167ff
Reviewed-on: http://review.couchbase.org/24813
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>