resource/aws_cloudwatch_log_stream: Prevent early state removal

References:
* hashicorp#11611

The AWS logs service has eventual consistency considerations. The
`aws_cloudwatch_log_stream` resource immediately tries to read a stream
after creation. If the stream is not found, the logs service returns a 200
OK with an empty list of streams. Since no streams are present, the
`aws_cloudwatch_log_stream` resource removes the created resource from
state, leading to a "produced an unexpected new value for was present,
but now absent" error.

With the changes in this commit, the empty list of streams in the response
for the newly created resource will result in a NotFoundError being returned
and a retry of the read request. A subsequent retry should hopefully be
successful, leading to the state being preserved.

Output from acceptance testing:

```
make testacc TEST=./aws TESTARGS='-run=TestAccAWSCloudWatchLogStream_'
...
--- PASS: TestAccAWSCloudWatchLogStream_disappears_LogGroup (16.77s)
--- PASS: TestAccAWSCloudWatchLogStream_disappears (19.55s)
--- PASS: TestAccAWSCloudWatchLogStream_basic (19.91s)
```

aws/resource_aws_cloudwatch_log_stream.go

@@ -4,9 +4,11 @@ import (
 	"fmt"
 	"log"
 	"regexp"
+	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/service/cloudwatchlogs"
+	"github.com/hashicorp/terraform-plugin-sdk/helper/resource"
 	"github.com/hashicorp/terraform-plugin-sdk/helper/schema"
 )
 
@@ -60,7 +62,24 @@ func resourceAwsCloudWatchLogStreamRead(d *schema.ResourceData, meta interface{}
 
 	group := d.Get("log_group_name").(string)
 
-	ls, exists, err := lookupCloudWatchLogStream(conn, d.Id(), group, nil)
+	var ls *cloudwatchlogs.LogStream
+	var exists bool
+
+	err := resource.Retry(2*time.Minute, func() *resource.RetryError {
+		var err error
+		ls, exists, err = lookupCloudWatchLogStream(conn, d.Id(), group, nil)
+		if err != nil {
+			return resource.NonRetryableError(err)
+		}
+		if d.IsNewResource() && !exists {
+			return resource.RetryableError(&resource.NotFoundError{})
+		}
+		return nil
+	})
+	if isResourceTimeoutError(err) {
+		ls, exists, err = lookupCloudWatchLogStream(conn, d.Id(), group, nil)
+	}
+
 	if err != nil {
 		if !isAWSErr(err, cloudwatchlogs.ErrCodeResourceNotFoundException, "") {
 			return err