provider/aws: Rename zone_id to hosted_zone_id in aws_db_instance

Also added a test to prove that the computed value gets set

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDBInstance_basic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/11/10 06:26:22 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSDBInstance_basic -timeout 120m
=== RUN   TestAccAWSDBInstance_basic
--- PASS: TestAccAWSDBInstance_basic (634.33s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws634.347s
```

builtin/providers/aws/resource_aws_db_instance.go

@@ -236,7 +236,7 @@ func resourceAwsDbInstance() *schema.Resource {
 				Computed: true,
 			},
 
-			"zone_id": &schema.Schema{
+			"hosted_zone_id": &schema.Schema{
 				Type:     schema.TypeString,
 				Computed: true,
 			},
@@ -674,7 +674,7 @@ func resourceAwsDbInstanceRead(d *schema.ResourceData, meta interface{}) error {
 	if v.Endpoint != nil {
 		d.Set("port", v.Endpoint.Port)
 		d.Set("address", v.Endpoint.Address)
-		d.Set("zone_id", v.Endpoint.HostedZoneId)
+		d.Set("hosted_zone_id", v.Endpoint.HostedZoneId)
 		if v.Endpoint.Address != nil && v.Endpoint.Port != nil {
 			d.Set("endpoint",
 				fmt.Sprintf("%s:%d", *v.Endpoint.Address, *v.Endpoint.Port))

builtin/providers/aws/resource_aws_db_instance_test.go

@@ -46,6 +46,7 @@ func TestAccAWSDBInstance_basic(t *testing.T) {
 						"aws_db_instance.bar", "username", "foo"),
 					resource.TestCheckResourceAttr(
 						"aws_db_instance.bar", "parameter_group_name", "default.mysql5.6"),
+					resource.TestCheckResourceAttrSet("aws_db_instance.bar", "hosted_zone_id"),
 				),
 			},
 		},