Don't try to expand true and false atoms in memcached config.

Change-Id: I89193f84b92f121727682ace35b3736b3967ad90
Reviewed-on: http://review.couchbase.org/38283
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>