MB-3948: correct calculation of RAM quota

We must use number of active servers

Change-Id: I9925d08d166bb337b738da204eb34f5b44c13d4e
Reviewed-on: http://review.couchbase.org/27393
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>