Corrected statname for bucket's memquota

- Using cbepctl for setting mem_low_wat,
and mem_high_wat breaks because ep_max_data_size
is used for bucket's memory quota which doesn't
exist anymore.
- Corrected stat: ep_max_size

Change-Id: If22ec8f43015c9fd87ad50674704a18089a7ee60
Reviewed-on: http://review.couchbase.org/32892
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>