Description of problem:
Tempest smoke test expects that the external network has only on IPv4 subnet, however in some corner cases there is a

Version-Release number of selected component (if applicable):
RHOSP13z9

How reproducible:
always

Steps to Reproduce:
1. Create external network for tempest and two IPv4 subnets on it
2. Run tempest run --smoke


Actual results:
test fails

Expected results:
tests finihses

Additional info:

tempest.scenario.test_network_basic_ops.TestNetworkBasicOps.test_network_basic_ops[compute,id-f323b3ba-82f8-4db7-8ea6-6a895869ec49,netw
ork,smoke]
---------------------------------------------------------------------------------------------------------------------------------------
----------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):
      File "/usr/lib/python2.7/site-packages/tempest/common/utils/__init__.py", line 88, in wrapper
        return f(*func_args, **func_kwargs)
      File "/usr/lib/python2.7/site-packages/tempest/scenario/test_network_basic_ops.py", line 409, in test_network_basic_ops
        self._check_network_external_connectivity()
      File "/usr/lib/python2.7/site-packages/tempest/scenario/test_network_basic_ops.py", line 342, in _check_network_external_connecti
vity
        "Found %d IPv4 subnets" % len(v4_subnets))
      File "/usr/lib/python2.7/site-packages/testtools/testcase.py", line 350, in assertEqual
        self.assertThat(observed, matcher, message)
      File "/usr/lib/python2.7/site-packages/testtools/testcase.py", line 435, in assertThat
        raise mismatch_error
    testtools.matchers._impl.MismatchError: 1 != 3: Found 3 IPv4 subnets