os_network: Allow freeform provider_network_type string

Prescribing types is not necessary as the underlying shade library
does not do so, and the Neutron API will inform us if a disallowed
or non existent type is used.

Fixes #20830

lib/ansible/modules/cloud/openstack/os_network.py

@@ -73,7 +73,6 @@ options:
    provider_network_type:
      description:
         - The type of physical network that maps to this network resource.
-     choices: ['flat', 'vlan', 'vxlan', 'gre', 'uplink']
      required: false
      default: None
      version_added: "2.1"
@@ -172,8 +171,7 @@ def main():
         admin_state_up=dict(default=True, type='bool'),
         external=dict(default=False, type='bool'),
         provider_physical_network=dict(required=False),
-        provider_network_type=dict(required=False, default=None,
+        provider_network_type=dict(required=False),
-                                   choices=['flat', 'vlan', 'vxlan', 'gre', 'uplink']),
         provider_segmentation_id=dict(required=False),
         state=dict(default='present', choices=['absent', 'present']),
         project=dict(default=None)