Add support for Elasticache Redis

[bmonkman]

Add terraform to enable support of Elasticache Redis.
As usual, let's try to use a well-supported existing TF module as the basis for this.

If this is done after memcached, the code should be reused. If it's done before it should take into account that we also want to support memcached.

[bmonkman]

Related to #168

[sshi100]

After investigation, I prefer to use this module, which supports both Memcache and Redis.

[bmonkman]

The aws_elasticache_cluster resource will make a cluster for memcached but it only supports a single instance group for redis. For Redis you should use a replication group. I would suggest using the cloudposse modules instead of just the provider.

[sshi100]

Decision: combine Cloudposse Redis and Memcached as a single module with selection of redis or memcache by user:

• https://registry.terraform.io/modules/cloudposse/elasticache-memcached/aws/latest
• https://registry.terraform.io/modules/cloudposse/elasticache-redis/aws/latest

[sshi100]

Support both Redis and Memcached, ready for go backend to use.

PRs:

• Module: 167 add cache terraform-aws-zero#32
• Infrastructure: 167 add cache #177
• Backend(go): 160 add cache zero-backend-go#55