MB-7719 disable 'Rebalance' button if the samples are loading

Change-Id: I258cdadbedc343e1bf6d0fcf5c91f8cee17f37bb
Reviewed-on: http://review.couchbase.org/27476
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Pavel Blagodov <stochmail@gmail.com>
Tested-by: Pavel Blagodov <stochmail@gmail.com>