adding option to create a route to nat gateway in database subnets

Added SSM and EC2 VPC endpoints

Added possibility to control creation of elasticache and redshift subnet groups

Added IGW route for DB subnets (based on #179)

Fix for the error: module.vpc.aws_redshift_subnet_group.redshift: onl…

Fix for the error: module.vpc.aws_redshift_subnet_group.redshift: only lowercase alphanumeric characters and hyphens allowed in name

Read more: https://github.com/terraform-aws-modules/terraform-aws-vpc/issues/180

* Added amazon_side_asn to vpn_gateway

* change to Amazon default ASN (as per API) (#176)

https://docs.aws.amazon.com/cli/latest/reference/ec2/create-vpn-gateway.html

* add vars for custom subnet and route table names

* revert db suffix to "db"

* Add secondary CIDR block support using a local variable to derive the vpc id to ensure the CIDR block operations are applied before the CIDR operations

* Add secondary cidr block outputs to module output

* Add the wonderful examples from matthiasr's PR located at https://github.com/terraform-aws-modules/terraform-aws-vpc/pull/162 all credit goes to them for this wonderful example

* From copy and paste accidentally used variable name that differed from these variables

* Resolve typo in secondary_cidr_blocks

* Fixed README formatting

* Followups for #161

* Added local.vpc_id with description

* Added support for IPv6 to VPC

* Provide separate route tables for db/elasticache/redshift

* Added example for saperate routes

* Updated PR with suggestions

* Make redshift to use separate subnet route table also

* More cleanup and updates

* Fixed one more spelling mistake

just a typo in the docs and in the
public_subnet_tags in the simple example

* Allow tags override for all resources (fix for #138)

* Added tags for all resources which were missing them

Fixed creation of aws_vpc_endpoint_route_table_association when intra_subnets are not set (fixes 137)

* Adding Lambda subnets for serverless functions that need to communicate
within the VPC

* Switched to "infra_subnet" for a private subnet without NAT gateway.

* Fixing typo - removing last "lambda" ref.

* Added README docs on intra subnets, and added it to complete VPC
example.  Fixed naming.

* Initial commit for issue #117

* Adding documentation for NAT Gateway creation

* Adding `enable_nat_gateway` to the documentation

* Updating README.md with `one_nat_gateway_per_az` to be 'true' by default

* Reverted changes back to `one_nat_gateway_per_az` as 'false'