Allow VLAN 0 for vlan segments

[wkleinhenz]

Im having some issue defining a VLAN segment with the nsxt_policy_vlan_segment resouce using the following definition `resource "nsxt_policy_vlan_segment" "vlan-uplink" {
display_name = "VLAN-UPLINK"
description = "VLAN-UPLINK"
transport_zone_path = data.nsxt_policy_transport_zone.std-vlan-01.path
vlan_ids = ["0"]

}`

the above fails with the error Error: invalid VLAN ID 0
i saw issue #128 was fixed with pull request #131 but thats focused on the manager side of things

if needed i can provide more information if needed