Create an app-essentials module

[rahearn]

Allow for including a single module for most cloud.gov apps. Example use:

module "app" {
  source = "this-github-repo//app"

  s3 = ["bucket-name1" = "desired-plan-name1", "bucket-name2" = "desired-plan-name2"]
  db = ["db-service-name" = "desired-plan-name"]

  cf_org_name      = var.cf_org_name
  cf_space_name    = var.cf_space_name
}