Introduce global resource prefix

[bobbyiliev]

As Kay suggested here: #13 (review)

Would it be possible to have a variable name_prefix so that for people can specify a unique name for the following?

1. cluster_name
2. environment
3. vpc_name
4. bucket_name
5. service_account_name
6. db_identifier

Otherwise, I think people in the same org trying this out can run into an error due to naming collision.

A popular prefix variable name in other TF modules is namespace:

variable "namespace" {
  type        = string
  description = "String used for prefix resources."
}

We should be careful and make sure that the string length is limited so that users don't use too long of a name.