[ElastiCache] Users should be taggable and should be valid for list_tags_for_resource

[jflim]

https://docs.aws.amazon.com/AmazonElastiCache/latest/APIReference/API_CreateUser.html

In addition list_tags_for_resource currently filters resources with the regex of r"^arn:aws:elasticache:.*:[0-9]*:(cluster|snapshot):.*$"