provider/aws: Support Import of `aws_db_instance`

JUst needed some rejigging of the skip_final_snapshot work as that isn't
returned by the API and skipping it means the destroy fails due to
missing final_snapshot_identifier

```
% make testacc TEST=./builtin/providers/aws
% TESTARGS='-run=TestAccAWSDBInstance_'                              ✹ ✭
==> Checking that code complies with gofmt requirements...
/Users/stacko/Code/go/bin/stringer
go generate $(go list ./... | grep -v /vendor/)
2016/07/07 15:28:31 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDBInstance_
-timeout 120m
=== RUN   TestAccAWSDBInstance_importBasic
--- PASS: TestAccAWSDBInstance_importBasic (588.70s)
=== RUN   TestAccAWSDBInstance_basic
--- PASS: TestAccAWSDBInstance_basic (595.71s)
=== RUN   TestAccAWSDBInstance_kmsKey
--- PASS: TestAccAWSDBInstance_kmsKey (726.46s)
=== RUN   TestAccAWSDBInstance_optionGroup
--- PASS: TestAccAWSDBInstance_optionGroup (681.78s)
=== RUN   TestAccAWSDBInstance_iops_update
--- PASS: TestAccAWSDBInstance_iops_update
(590.81s)
```

Please note that I cannot run the enhanced monitoring test in my
environment as I have already got it attached to an IAM role. Running
that test gives me this result:

```

```

builtin/providers/aws/import_aws_db_instance_test.go

@@ -0,0 +1,30 @@
+package aws
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform/helper/resource"
+)
+
+func TestAccAWSDBInstance_importBasic(t *testing.T) {
+	resourceName := "aws_db_instance.bar"
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSDBInstanceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccAWSDBInstanceConfig,
+			},
+
+			resource.TestStep{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+				ImportStateVerifyIgnore: []string{
+					"password", "skip_final_snapshot"},
+			},
+		},
+	})
+}

builtin/providers/aws/resource_aws_db_instance.go

@@ -22,6 +22,9 @@ func resourceAwsDbInstance() *schema.Resource {
 		Read:   resourceAwsDbInstanceRead,
 		Update: resourceAwsDbInstanceUpdate,
 		Delete: resourceAwsDbInstanceDelete,
+		Importer: &schema.ResourceImporter{
+			State: schema.ImportStatePassthrough,
+		},
 
 		Schema: map[string]*schema.Schema{
 			"name": &schema.Schema{
@@ -633,6 +636,7 @@ func resourceAwsDbInstanceRead(d *schema.ResourceData, meta interface{}) error {
 	}
 
 	d.Set("name", v.DBName)
+	d.Set("identifier", v.DBInstanceIdentifier)
 	d.Set("username", v.MasterUsername)
 	d.Set("engine", v.Engine)
 	d.Set("engine_version", v.EngineVersion)
@@ -753,10 +757,13 @@ func resourceAwsDbInstanceDelete(d *schema.ResourceData, meta interface{}) error
 
 	opts := rds.DeleteDBInstanceInput{DBInstanceIdentifier: aws.String(d.Id())}
 
-	skipFinalSnapshot := d.Get("skip_final_snapshot").(bool)
-	opts.SkipFinalSnapshot = aws.Bool(skipFinalSnapshot)
+	skipFinalSnapshot, exists := d.GetOk("skip_final_snapshot")
+	if !exists {
+		skipFinalSnapshot = true
+	}
+	opts.SkipFinalSnapshot = aws.Bool(skipFinalSnapshot.(bool))
 
-	if !skipFinalSnapshot {
+	if skipFinalSnapshot == false {
 		if name, present := d.GetOk("final_snapshot_identifier"); present {
 			opts.FinalDBSnapshotIdentifier = aws.String(name.(string))
 		} else {