upgrade memcached engine config

Because ep-engine folks want to change static_config of ep-engine.

Change-Id: I40613475f17794170b666fdda55e2b47c2f62fcd
Reviewed-on: http://review.couchbase.org/41265
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>