provider/aws: fix Elastic Beanstalk cname_prefix

[dharrisio]

Fixes an issue where the cname_prefix attribute isn't correctly read
in some regions.

Previously environments in some regions would have recurring plans if the cname_prefix attribute was used. Running terraform plan after a terraform apply has this diff

-/+ aws_elastic_beanstalk_environment.default
    all_settings.#:          "94" => "<computed>"
    application:             "tf-test-cname-output" => "tf-test-cname-output"
    autoscaling_groups.#:    "1" => "<computed>"
    cname:                   "tf-test-cname-output-env.fvixezmt3v.ap-southeast-2.elasticbeanstalk.com" => "<computed>"
    cname_prefix:            "" => "region-prefix" (forces new resource)
    instances.#:             "1" => "<computed>"
    launch_configurations.#: "1" => "<computed>"
    load_balancers.#:        "1" => "<computed>"
    name:                    "tf-test-cname-output-env" => "tf-test-cname-output-env"
    queues.#:                "0" => "<computed>"
    setting.#:               "0" => "<computed>"
    solution_stack_name:     "64bit Amazon Linux running Python" => "64bit Amazon Linux running Python"
    tier:                    "WebServer" => "WebServer"
    triggers.#:              "0" => "<computed>"
    wait_for_ready_timeout:  "10m" => "10m"

Tests

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSBeanstalkEnv_basic -timeout 120m
=== RUN   TestAccAWSBeanstalkEnv_basic
--- PASS: TestAccAWSBeanstalkEnv_basic (395.00s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    395.018s

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSBeanstalkEnv_cname_prefix -timeout 120m
=== RUN   TestAccAWSBeanstalkEnv_cname_prefix
--- PASS: TestAccAWSBeanstalkEnv_cname_prefix (344.87s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    344.891s

[stack72]

Hi @dharrisio

This looks good to me! Thanks for the continued work on ElasticBeanstalk!

p.

[ghost]

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