Description of problem:

Version-Release number of selected component (if applicable):
RHOS_TRUNK-16.0-RHEL-8-20200324.n.0

How reproducible: 100%

Steps to Reproduce:
1. Deploy OSP 16 RHOS_TRUNK-16.0-RHEL-8-20200324.n.0 with ML2/OVS and DVR enabled
2. Create Nova instance (cirros is fine)
3. Associate floating IP to Nova instance
4. Create security group rules to allow incoming traffic
5. Try to connect to the Nova instance via the FIP

Actual results:
A connection to the VM cannot be established.

Expected results:
Connection to VM should be established.

Additional info:
This issue was detected in downstream Octavia CI by an ML2/OVS + DVR job. Octavia traffic scenario tests consistently failed (5 times) with compose RHOS_TRUNK-16.0-RHEL-8-20200324.n.0.  Older compose RHOS_TRUNK-16.0-RHEL-8-20200226.n.1 works well -- the CI job passed 5 times.

Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/tempest/test.py", line 188, in setUpClass
    six.reraise(etype, value, trace)
  File "/usr/lib/python3.6/site-packages/six.py", line 675, in reraise
    raise value
  File "/usr/lib/python3.6/site-packages/tempest/test.py", line 181, in setUpClass
    cls.resource_setup()
  File "/usr/lib/python3.6/site-packages/octavia_tempest_plugin/tests/scenario/v2/test_traffic_ops.py", line 38, in resource_setup
    super(TrafficOperationsScenarioTest, cls).resource_setup()
  File "/usr/lib/python3.6/site-packages/octavia_tempest_plugin/tests/test_base.py", line 617, in resource_setup
    cls.webserver1_ipv6, cls.lb_member_1_subnet_prefix)
  File "/usr/lib/python3.6/site-packages/octavia_tempest_plugin/tests/test_base.py", line 833, in _enable_ipv6_nic_webserver
    linux_client.validate_authentication()
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/utils/linux/remote_client.py", line 60, in wrapper
    six.reraise(*original_exception)
  File "/usr/lib/python3.6/site-packages/six.py", line 675, in reraise
    raise value
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/utils/linux/remote_client.py", line 33, in wrapper
    return function(self, *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/utils/linux/remote_client.py", line 116, in validate_authentication
    self.ssh_client.test_connection_auth()
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/ssh.py", line 209, in test_connection_auth
    connection = self._get_ssh_connection()
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/ssh.py", line 121, in _get_ssh_connection
    password=self.password)
tempest.lib.exceptions.SSHTimeout: Connection to the 10.0.0.227 via SSH timed out.
User: cirros, Password: None