Update elasticache model to the latest version

Summary of changes:

* REMOVED: Key removed in CreateCacheCluster.CacheNodeType: required
* REMOVED: Key removed in CreateCacheCluster.Engine: required
* REMOVED: Key removed in CreateCacheCluster.NumCacheNodes: required
* DOCS: DeleteReplicationGroup operation documentation has changed.
* ADD: New argument added to operation DeleteReplicationGroup: RetainPrimaryCluster
Merge branch 'update-elasticache-model' into develop

* update-elasticache-model:
  Update elasticache model to the latest version