Add lb list extract

Author: hailun.huang

Diff for: openstack/loadbalancer/v2/quotas/results.go

@@ -24,6 +24,27 @@ type QuotaPage struct {
 	pagination.LinkedPageBase
 }
 
+func (r QuotaPage) NextPageURL() (string, error) {
+	var s struct {
+		Links []gophercloud.Link `json:"quota_links"`
+	}
+	err := r.ExtractInto(&s)
+	if err != nil {
+		return "", err
+	}
+	return gophercloud.ExtractNextURL(s.Links)
+}
+
+// IsEmpty checks whether a FlavorPage struct is empty.
+func (r QuotaPage) IsEmpty() (bool, error) {
+	if r.StatusCode == 204 {
+		return true, nil
+	}
+
+	is, err := ExtractQuotas(r)
+	return len(is) == 0, err
+}
+
 func ExtractQuotas(r pagination.Page) ([]Quota, error) {
 	var s struct {
 		Quotas []Quota `json:"quotas"`