Networking: expose subnets in network data source

[kayrus]

Can be used to solve the #1150 issue. Less efficient way to list and filter external network subnets:

variable "pattern" {}
  
data "openstack_networking_network_v2" "network" {
  name = "external-network"
}

data "openstack_networking_subnet_v2" "subnet" {
  count     = length(data.openstack_networking_network_v2.network.subnets)
  subnet_id = data.openstack_networking_network_v2.network.subnets[count.index]
}

output "subnet_ids" {
  value = [for x in data.openstack_networking_subnet_v2.subnet : x.id if length(regexall(var.pattern, x.name)) > 0]
}

output "subnet_names" {
  value = [for x in data.openstack_networking_subnet_v2.subnet : x.name if length(regexall(var.pattern, x.name)) > 0]
}

[theopenlab-ci]

Build succeeded.

• terraform-provider-openstack-unittest : SUCCESS in 3m 12s
• terraform-provider-openstack-acceptance-test : SUCCESS in 2h 35m 43s

[ozerovandrei]

LGTM