Rename "Interface" to "Availability".

Interface is what the identity v3 docs call it, but that collides with an
important concept from Go-the-language. Renaming it to "Availability" because
that's more descriptive.
diff --git a/openstack/identity/v3/endpoints/results.go b/openstack/identity/v3/endpoints/results.go
index 940eebc..bd7a013 100644
--- a/openstack/identity/v3/endpoints/results.go
+++ b/openstack/identity/v3/endpoints/results.go
@@ -9,12 +9,12 @@
 
 // Endpoint describes the entry point for another service's API.
 type Endpoint struct {
-	ID        string                `json:"id"`
-	Interface gophercloud.Interface `json:"interface"`
-	Name      string                `json:"name"`
-	Region    string                `json:"region"`
-	ServiceID string                `json:"service_id"`
-	URL       string                `json:"url"`
+	ID           string                   `json:"id"`
+	Availability gophercloud.Availability `json:"interface"`
+	Name         string                   `json:"name"`
+	Region       string                   `json:"region"`
+	ServiceID    string                   `json:"service_id"`
+	URL          string                   `json:"url"`
 }
 
 // EndpointList contains a page of Endpoint results.