provider/aws: Retry creation of IAM role depending on new IAM user

[dtolnay]

Fixes #4709.

[catsby]

checks out, thanks @dtolnay !

Ran the following:

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSRole_ -timeout 120m
=== RUN   TestAccAWSRole_basic
--- PASS: TestAccAWSRole_basic (8.07s)
=== RUN   TestAccAWSRole_namePrefix
--- PASS: TestAccAWSRole_namePrefix (9.63s)
=== RUN   TestAccAWSRole_testNameChange
--- PASS: TestAccAWSRole_testNameChange (19.45s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    37.172s
Test:

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_b -timeout 120m
=== RUN   TestAccAWSInstance_basic
ma--- PASS: TestAccAWSInstance_basic (123.26s)
=== RUN   TestAccAWSInstance_blockDevices
--- PASS: TestAccAWSInstance_blockDevices (81.44s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    204.718s
Test:

[ghost]

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