interrupt test if rebalance makes no progress after 2h

Change-Id: If9b4dbb39c8576d2d9cd4b74a4316a2e7e7e0631
Reviewed-on: http://review.couchbase.org/39819
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>