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/errors.go b/openstack/identity/v3/endpoints/errors.go
index a8b00d4..854957f 100644
--- a/openstack/identity/v3/endpoints/errors.go
+++ b/openstack/identity/v3/endpoints/errors.go
@@ -7,8 +7,8 @@
 }
 
 var (
-	// ErrInterfaceRequired is reported if an Endpoint is created without an Interface.
-	ErrInterfaceRequired = requiredAttribute("an interface")
+	// ErrAvailabilityRequired is reported if an Endpoint is created without an Availability.
+	ErrAvailabilityRequired = requiredAttribute("an availability")
 
 	// ErrNameRequired is reported if an Endpoint is created without a Name.
 	ErrNameRequired = requiredAttribute("a name")