add elasticache / elasticache group creation as an option

[while1eq1]

Changes introduces by this PR

When building a VPC you may require to add Elasticache instances into your infrastructure. This might require isolated private subnet groups apart from the existing ones. Below are the changes introduced by this PR.

• Add the elasticache private elasticache subnets
• Create elasticache subnet group
• Associate the subnets with private route tables

Provide output for the following:

• A list of elasticache subnet ids
• The id of the elasticache subnet group usage

module "vpc" {
...
  elasticache_subnets   = ["10.0.90.0/24", "10.0.91.0/24", "10.0.92.0/24"]
....
}

Terraform will only provision these resources only when the variable elasticache_subnets is provided.

[antonbabenko]

LGTM, thank you.

[antonbabenko]

https://github.com/terraform-community-modules/tf_aws_vpc/releases/tag/v1.0.4 - new release has been taged