Add docs for TF vars introduced PR 4239 (#5201)

contrib/terraform/openstack/README.md

@@ -248,6 +248,11 @@ For your cluster, edit `inventory/$CLUSTER/cluster.tfvars`.
 |`k8s_allowed_remote_ips` | List of CIDR allowed to initiate a SSH connection, empty by default |
 |`worker_allowed_ports` | List of ports to open on worker nodes, `[{ "protocol" = "tcp", "port_range_min" = 30000, "port_range_max" = 32767, "remote_ip_prefix" = "0.0.0.0/0"}]` by default |
 |`wait_for_floatingip` | Let Terraform poll the instance until the floating IP has been associated, `false` by default. |
+|`node_root_volume_size_in_gb` | Size of the root volume for nodes, 0 to use ephemeral storage |
+|`master_root_volume_size_in_gb` | Size of the root volume for masters, 0 to use ephemeral storage |
+|`gfs_root_volume_size_in_gb` | Size of the root volume for gluster, 0 to use ephemeral storage |
+|`etcd_root_volume_size_in_gb` | Size of the root volume for etcd nodes, 0 to use ephemeral storage |
+|`bastion_root_volume_size_in_gb` | Size of the root volume for bastions, 0 to use ephemeral storage |
 
 #### Terraform state files