zones: introduce ZoneIterator, implemented by Zones

Signed-off-by: Alejandro Mery <amery@jpi.io>

pkg/zones/zones.go

@@ -13,6 +13,7 @@ import (
 var (
 	_ MachineIterator = (*Zone)(nil)
 	_ MachineIterator = (*Zones)(nil)
+	_ ZoneIterator    = (*Zones)(nil)
 )
 
 // A MachineIterator is a set of Machines we can iterate on
@@ -20,6 +21,11 @@ type MachineIterator interface {
 	ForEachMachine(func(*Machine) bool)
 }
 
+// A ZoneIterator is a set of Zones we can iterate on
+type ZoneIterator interface {
+	ForEachZone(func(*Zone) bool)
+}
+
 // Zone represents one zone in a cluster
 type Zone struct {
 	zones *Zones