beta: rename memcached object cache

In production 'memcached-pecl' is referenced from CommonSettings.php for
the 'mysql-multiwrite' object caches.  But on beta 'memcached-pecl' does
not exist and MediaWiki fallback to $wgMemcachedServers which is
127.0.0.1:11211, since we migrated from twemproxy to nutcracker, it is
no more available.

This patch makes beta configuration similiar to the production one.
Will refactor in a follow up patch.

Change-Id: Idf4bb69a0580817425958bd857edd247eca7d671