provider/aws: Improved IAM Role description tests & validation

[Ninir]

Description

This augments #14208 to add acceptance tests, adding validation. (I also started the work for the description, but @minamijoyo did it fast 😄 )

Acceptance tests create the role with the description, then update it.
This work also adds more randomization to iam role tests.

Tests

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSIAMRole_importBasic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/04 21:10:03 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSIAMRole_importBasic -timeout 120m
=== RUN   TestAccAWSIAMRole_importBasic
--- PASS: TestAccAWSIAMRole_importBasic (17.14s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	17.170s

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSRole_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/04 22:23:31 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSRole_ -timeout 120m
=== RUN   TestAccAWSRole_basic
--- PASS: TestAccAWSRole_basic (15.90s)
=== RUN   TestAccAWSRole_basicWithDescription
--- PASS: TestAccAWSRole_basicWithDescription (38.47s)
=== RUN   TestAccAWSRole_namePrefix
--- PASS: TestAccAWSRole_namePrefix (15.29s)
=== RUN   TestAccAWSRole_testNameChange
--- PASS: TestAccAWSRole_testNameChange (35.00s)
=== RUN   TestAccAWSRole_badJSON
--- PASS: TestAccAWSRole_badJSON (0.00s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	104.691s

[stack72]

Nice work as always @Ninir :)

[ghost]

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