aws: Implement naming changes for aws_elb_service_account

[radeksimko]

Implementing name changes as discussed in #8221 (comment)

cc @apparentlymart

Acceptance tests

TF_ACC=1 go test ./builtin/providers/aws -v -run=AWSElbServiceAccount -timeout 120m
=== RUN   TestAccAWSElbServiceAccount_basic
--- PASS: TestAccAWSElbServiceAccount_basic (19.94s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    19.961s

[apparentlymart]

LGTM! Thanks for indulging me 😀

[mitchellh]

LGTM, I'm going to merge this right away since we're planning on cutting a 0.7.1 and don't want to otherwise introduce a BC.

[ghost]

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