Recent NSX-T NSGroup API change cause error in NSGroup cleanup

We call nsxt_cleanup during unstack to cleanup the backend resources created from our neutron plugin. However, the recent API change in NSGroup causes exception in Firewall Section cleanup.

stack@htb-1n-eng-dhcp8:~/devstack$ ./unstack.sh
Overriding Configuration Defaults
Loading plugin settings
Number of OS Firewall Sections to be deleted: 0
Traceback (most recent call last):
  File "/opt/stack/vmware-nsx/devstack/tools/nsxt_cleanup.py", line 402, in <module>
    nsx_client.cleanup_all()
  File "/opt/stack/vmware-nsx/devstack/tools/nsxt_cleanup.py", line 381, in cleanup_all
    self.cleanup_os_ns_groups()
  File "/opt/stack/vmware-nsx/devstack/tools/nsxt_cleanup.py", line 298, in cleanup_os_ns_groups
    ns_groups = self.get_os_ns_groups()
  File "/opt/stack/vmware-nsx/devstack/tools/nsxt_cleanup.py", line 291, in get_os_ns_groups
    ns_groups = self.get_ns_groups()
  File "/opt/stack/vmware-nsx/devstack/tools/nsxt_cleanup.py", line 285, in get_ns_groups
    return response.json()['results']
KeyError: 'results'