MB-8094: sleep 10sec before rebalance(mixed cluster)

Change-Id: Iec5a35114b1a94513f312582a473abe03c68b1f3
Reviewed-on: http://review.couchbase.org/25728
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.salooja@globallogic.com>