MB-10320 Disallow editing server groups during rebalance on UI.

Change-Id: I9eb63c5474b56bbcc025a00742c66d7c83a24af0
Reviewed-on: http://review.couchbase.org/34218
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>