MB-100 Ignore some irrelevant settings for memcached buckets.

Change-Id: I4fb450c23b888b6715b47c045824754330a54d47
Reviewed-on: http://review.couchbase.org/26630
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>