resource/aws_cloudwatch_event_rule: Prevent multiple names on creation

[bflad]

Fixes #4547

Changes proposed in this pull request:

• Only perform name_prefix/generated name logic once on creation

Create -> Update -> Read strikes again! Similar as observed in the issue, the daily acceptance testing has been leaving dangling rules:

$ aws events list-rules | jq '.Rules[] | .Name'
"tf-acc-cw-event-rule-prefix-20180426162323934100000002"
"tf-acc-cw-event-rule-prefix-20180426162514120400000002"
"tf-acc-cw-event-rule-prefix-20180427053026579200000002"
"tf-acc-cw-event-rule-prefix-20180428060328552200000002"
"tf-acc-cw-event-rule-prefix-20180429060519520300000002"
"tf-acc-cw-event-rule-prefix-20180501053203121800000002"
"tf-acc-cw-event-rule-prefix-20180502053210453100000002"
"tf-acc-cw-event-rule-prefix-20180503060601159900000002"
"tf-acc-cw-event-rule-prefix-20180504063840134000000002"
"tf-acc-cw-event-rule-prefix-20180505054500305900000002"
"tf-acc-cw-event-rule-prefix-20180506060233773100000002"
"tf-acc-cw-event-rule-prefix-20180508053138002100000002"
"tf-acc-cw-event-rule-prefix-20180509053336875600000002"
"tf-acc-cw-event-rule-prefix-20180510064223367500000002"
"tf-acc-cw-event-rule-prefix-20180512060518989300000002"
"tf-acc-cw-event-rule-prefix-20180514053257185000000002"
"tf-acc-cw-event-rule-prefix-20180515054306224300000002"
"tf-acc-cw-event-rule-prefix-20180516062014304000000002"
"tf-acc-cw-event-rule-prefix-20180517053608965300000002"

After code update and re-running this test:

$ make testacc TEST=./aws TESTARGS='-run=TestAccAWSCloudWatchEventRule_prefix'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSCloudWatchEventRule_prefix -timeout 120m
=== RUN   TestAccAWSCloudWatchEventRule_prefix
--- PASS: TestAccAWSCloudWatchEventRule_prefix (18.37s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	18.403s

The dangling name is not created.

[mbfrahry]

LGTM

[bflad]

This has been released in version 1.20.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!