blockstorage: add helper to return available AZs

A similar utility exists for the compute API. Introduce one for the
block storage API.

Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

Author: stephenfin

openstack/blockstorage/v3/availabilityzones/utils.go

@@ -0,0 +1,35 @@
+package availabilityzones
+
+import (
+	"github.com/gophercloud/gophercloud"
+	"github.com/gophercloud/gophercloud/openstack/blockstorage/extensions/availabilityzones"
+)
+
+// ListAvailableAvailabilityZones is a convenience function that return a slice of available Availability Zones.
+func ListAvailableAvailabilityZones(client *gophercloud.ServiceClient) ([]string, error) {
+	var zones []string
+
+	allPages, err := availabilityzones.List(client).AllPages()
+	if err != nil {
+		return nil, err
+	}
+
+	availabilityZoneInfo, err := availabilityzones.ExtractAvailabilityZones(allPages)
+	if err != nil {
+		return nil, err
+	}
+
+	// This should always return at at least two AZs. By default, Nova will
+	// return an AZ for internal services (typically called 'internal') and AZ
+	// for (typically called 'nova'). We can obviously configure additional AZs
+	// and you can also configure the names of these default AZs with
+	// '[DEFAULT] internal_service_availability_zone' and
+	// '[DEFAULT] default_availability_zone', respectively.
+	for _, zone := range availabilityZoneInfo {
+		if zone.ZoneState.Available {
+			zones = append(zones, zone.ZoneName)
+		}
+	}
+
+	return zones, nil
+}