availability zones support for OpenStack

allow masters, nodes and gluster nodes (within each group) to be scheduled
on differents AZ.

contrib/terraform/openstack/modules/compute/variables.tf

@@ -1,5 +1,9 @@
 variable "cluster_name" {}
 
+variable "az_list" {
+  type = "list"
+}
+
 variable "number_of_k8s_masters" {}
 
 variable "number_of_k8s_masters_no_etcd" {}