MB-10320 Disallow updating server groups during rebalance.

Change-Id: I27f513e515b64ead0eb5ecc84486f8f9951dcfbd
Reviewed-on: http://review.couchbase.org/34217
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>