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/flavors/results.go b/openstack/db/v1/flavors/results.go
index f74f20c..0ba515c 100644
--- a/openstack/db/v1/flavors/results.go
+++ b/openstack/db/v1/flavors/results.go
@@ -22,6 +22,7 @@
 // Flavor records represent (virtual) hardware configurations for server resources in a region.
 type Flavor struct {
 	// The flavor's unique identifier.
+	// Contains 0 if the ID is not an integer.
 	ID int `json:"id"`
 
 	// The RAM capacity for the flavor.
@@ -32,6 +33,9 @@
 
 	// Links to access the flavor.
 	Links []gophercloud.Link
+
+	// The flavor's unique identifier as a string
+	StrID string `json:"str_id"`
 }
 
 // FlavorPage contains a single page of the response from a List call.