Compute v2: Allow correct handling of Image Metadata (#285)

The Compute v2 API expects JSON values for metadata with keys of
boot_from_volume and mappings. This commit changes the Metadata
type from map[string]string to map[string]interface{} to handle
these values.
diff --git a/openstack/compute/v2/images/results.go b/openstack/compute/v2/images/results.go
index 1090327..f9ebc69 100644
--- a/openstack/compute/v2/images/results.go
+++ b/openstack/compute/v2/images/results.go
@@ -46,7 +46,7 @@
 
 	Updated string
 
-	Metadata map[string]string
+	Metadata map[string]interface{}
 }
 
 // ImagePage contains a single page of results from a List operation.