MB-100 replace hardcoded MiB with define

Change-Id: I221eb2bc92e3199796695d6cb7c47cf4f9c19244
Reviewed-on: http://review.couchbase.org/29460
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>