Cross-az deployment for elasticache should be defaulted to on, and overridable by the user

AZ mode string should be set to 'cross-az' by default and overridable to 'single-az'

see AZ mode setting here: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-elasticache-cache-cluster.html

and possible values here:
https://docs.aws.amazon.com/AmazonElastiCache/latest/APIReference/API_CreateCacheCluster.html