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/acceptance/openstack/identity/v3/endpoint_test.go b/acceptance/openstack/identity/v3/endpoint_test.go
index b8c1dcc..14783b5 100644
--- a/acceptance/openstack/identity/v3/endpoint_test.go
+++ b/acceptance/openstack/identity/v3/endpoint_test.go
@@ -29,7 +29,7 @@
 		for _, endpoint := range endpoints3.AsEndpoints(page) {
 			t.Logf("Endpoint: %8s %10s %9s %s",
 				endpoint.ID,
-				endpoint.Interface,
+				endpoint.Availability,
 				endpoint.Name,
 				endpoint.URL)
 		}
@@ -66,8 +66,8 @@
 
 	// Enumerate the endpoints available for this service.
 	endpointResults, err := endpoints3.List(client, endpoints3.ListOpts{
-		Interface: gophercloud.InterfacePublic,
-		ServiceID: computeService.ID,
+		Availability: gophercloud.AvailabilityPublic,
+		ServiceID:    computeService.ID,
 	})
 
 	allEndpoints, err := gophercloud.AllPages(endpointResults)