Specifies whether the new nodes in this Memcached cache cluster are all created in a single Availability Zone or created across multiple Availability Zones.
Valid values: single-az | cross-az.
This option is only supported for Memcached cache clusters.
You cannot specify single-az if the Memcached cache cluster already has
cache nodes in different Availability Zones. If cross-az is specified, existing Memcached nodes remain in their current Availability Zone.
Only newly created nodes will be located in different Availability Zones. For instructions on how to move existing Memcached nodes to different Availability Zones, see the Availability Zone Considerations section of Cache Node Considerations for Memcached (http://docs.aws.amazon.com/AmazonElastiCache/latest/UserGuide/CacheNode.Memcached.html).