openstack: Add update subnet DHCP helper

(cherry picked from commit 3f711ff)

unit_tests/utilities/test_zaza_utilities_openstack.py

@@ -1500,6 +1500,20 @@ def test_configure_charmed_openstack_on_maas(self):
         self.configure_networking_charms.assert_called_once_with(
             'fakenetworkingdata', expect, use_juju_wait=False)
 
+    def test_update_subnet_dhcp(self):
+        neutron_client = mock.MagicMock()
+        openstack_utils.update_subnet_dhcp(
+            neutron_client, {'id': 'aId'}, True)
+        neutron_client.update_subnet.assert_called_once_with(
+            'aId',
+            {'subnet': {'enable_dhcp': True}})
+        neutron_client.reset_mock()
+        openstack_utils.update_subnet_dhcp(
+            neutron_client, {'id': 'aId'}, False)
+        neutron_client.update_subnet.assert_called_once_with(
+            'aId',
+            {'subnet': {'enable_dhcp': False}})
+
 
 class TestAsyncOpenstackUtils(ut_utils.AioTestCase):
 

zaza/openstack/utilities/openstack.py

@@ -1332,6 +1332,24 @@ def update_subnet_dns(neutron_client, subnet, dns_servers):
     neutron_client.update_subnet(subnet['id'], msg)
 
 
+def update_subnet_dhcp(neutron_client, subnet, enable_dhcp):
+    """Update subnet DHCP status.
+
+    :param neutron_client: Authenticated neutronclient
+    :type neutron_client: neutronclient.Client object
+    :param subnet: Subnet object
+    :type subnet: dict
+    :param enable_dhcp: Whether DHCP should be enabled or not
+    :type enable_dhcp: bool
+    """
+    msg = {
+        'subnet': {
+            'enable_dhcp': enable_dhcp,
+        }
+    }
+    neutron_client.update_subnet(subnet['id'], msg)
+
+
 def create_provider_router(neutron_client, project_id):
     """Create the provider router.