db: fix flavor id parsing (#197)

* Flavor IDs returned with instances are always strings
* Flavor IDs returned with flavors are integers or null, there is a
  str_id parameter that contains the ID in string format
diff --git a/openstack/db/v1/instances/testing/fixtures.go b/openstack/db/v1/instances/testing/fixtures.go
index 8603e98..79ee9c3 100644
--- a/openstack/db/v1/instances/testing/fixtures.go
+++ b/openstack/db/v1/instances/testing/fixtures.go
@@ -7,7 +7,6 @@
 
 	"github.com/gophercloud/gophercloud"
 	"github.com/gophercloud/gophercloud/openstack/db/v1/datastores"
-	"github.com/gophercloud/gophercloud/openstack/db/v1/flavors"
 	"github.com/gophercloud/gophercloud/openstack/db/v1/instances"
 	"github.com/gophercloud/gophercloud/testhelper/fixture"
 )
@@ -25,7 +24,7 @@
     "version": "5.6"
   },
   "flavor": {
-    "id": 1,
+    "id": "1",
     "links": [
       {
         "href": "https://my-openstack.com/v1.0/1234/flavors/1",
@@ -112,8 +111,8 @@
 var expectedInstance = instances.Instance{
 	Created: timeVal,
 	Updated: timeVal,
-	Flavor: flavors.Flavor{
-		ID: 1,
+	Flavor: instances.Flavor{
+		ID: "1",
 		Links: []gophercloud.Link{
 			{Href: "https://my-openstack.com/v1.0/1234/flavors/1", Rel: "self"},
 			{Href: "https://my-openstack.com/v1.0/1234/flavors/1", Rel: "bookmark"},