provider/aws: Support Import `aws_rds_cluster_instance` (#7522)

We were not setting all the values in the read Func. One other issue, we
were setting the *wrong* db_parameter_name value to state

```
make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSRDSClusterInstance_import'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSRDSClusterInstance_import -timeout 120m
=== RUN   TestAccAWSRDSClusterInstance_importBasic
--- PASS: TestAccAWSRDSClusterInstance_importBasic (1201.80s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws
1201.814s
```

builtin/providers/aws/import_aws_rds_cluster_instance_test.go

@@ -0,0 +1,29 @@
+package aws
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform/helper/acctest"
+	"github.com/hashicorp/terraform/helper/resource"
+)
+
+func TestAccAWSRDSClusterInstance_importBasic(t *testing.T) {
+	resourceName := "aws_rds_cluster_instance.cluster_instances"
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSClusterDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccAWSClusterInstanceConfig(acctest.RandInt()),
+			},
+
+			resource.TestStep{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
+		},
+	})
+}

builtin/providers/aws/resource_aws_rds_cluster_instance.go

@@ -17,6 +17,9 @@ func resourceAwsRDSClusterInstance() *schema.Resource {
 		Read:   resourceAwsRDSClusterInstanceRead,
 		Update: resourceAwsRDSClusterInstanceUpdate,
 		Delete: resourceAwsRDSClusterInstanceDelete,
+		Importer: &schema.ResourceImporter{
+			State: schema.ImportStatePassthrough,
+		},
 
 		Schema: map[string]*schema.Schema{
 			"identifier": &schema.Schema{
@@ -185,9 +188,12 @@ func resourceAwsRDSClusterInstanceRead(d *schema.ResourceData, meta interface{})
 	}
 
 	d.Set("publicly_accessible", db.PubliclyAccessible)
+	d.Set("cluster_identifier", db.DBClusterIdentifier)
+	d.Set("instance_class", db.DBInstanceClass)
+	d.Set("identifier", db.DBInstanceIdentifier)
 
 	if len(db.DBParameterGroups) > 0 {
-		d.Set("parameter_group_name", db.DBParameterGroups[0].DBParameterGroupName)
+		d.Set("db_parameter_group_name", db.DBParameterGroups[0].DBParameterGroupName)
 	}
 
 	// Fetch and save tags