feature(rds): add rds database subnets

[nyambati]

Changes introduces by this PR

When building a VPC you may require to add RDS 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 RDS private database subnets
• Create RDS subnet group
• Associate the subnets with private route tables
• Provide output for the following:
  • A list of database subnet ids
  • The id of the subnet group

usage

module "vpc" {
...
  database_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 database_subnets is provided.

[antonbabenko]

Also, please update README.

[nyambati]

@antonbabenko I have made the updates, It should be all good now. Thanks for the review.

[tfhartmann]

Would someone mind tagging a new release?

[antonbabenko]

Done. https://github.com/terraform-community-modules/tf_aws_vpc/releases/tag/v1.0.3