Improved documentation how to override the AWS environment variable

[aaronsteers]

In a recent demo, a user asked how to leverage an existing client VPC and existing subnets. The below sample code would have been useful if it were included in the documentation.

Important to note that this:

 module ___ ____ {
 ...
 environment = module.env.environment
 ...

...can be rewritten as:

 module ___ ____ {
 ...
  environment = {
    vpc_id          = module.env.environment.vpc_id
    aws_region      = module.env.environment.aws_region
    public_subnets  = module.env.environment.public_subnets
    private_subnets = module.env.environment.private_subnets
  }
 ...

... and that then each variable can be overridden or fed in from another source.