pyroute2 usage makes binding lib users fail on unit tests

[Irena Berezovsky]

when unit tests for kuryr binding lib users are executed on other than linux environment (MacOS/Win), it failes:
unittest2.loader._FailedTest.kuryr_kubernetes.tests.unit.test_os_vif_util
-------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):
    ImportError: Failed to import test module: kuryr_kubernetes.tests.unit.test_os_vif_util
    Traceback (most recent call last):
      File "/Users/irenab/Work/kuryr-kubernetes/.tox/py27/lib/python2.7/site-packages/unittest2/loader.py", line 456, in _find_test_path
        module = self._get_module_from_name(name)
      File "/Users/irenab/Work/kuryr-kubernetes/.tox/py27/lib/python2.7/site-packages/unittest2/loader.py", line 395, in _get_module_from_name
        __import__(name)
      File "kuryr_kubernetes/tests/unit/test_os_vif_util.py", line 26, in <module>
        from kuryr_kubernetes import os_vif_util as ovu
      File "kuryr_kubernetes/os_vif_util.py", line 19, in <module>
        from kuryr.lib.binding.drivers import utils as kl_utils
      File "/Users/irenab/Work/kuryr-kubernetes/.tox/py27/lib/python2.7/site-packages/kuryr/lib/binding/drivers/utils.py", line 15, in <module>
        from pyroute2.netlink.rtnl import ifinfmsg
      File "/Users/irenab/Work/kuryr-kubernetes/.tox/py27/lib/python2.7/site-packages/pyroute2/netlink/rtnl/ifinfmsg/__init__.py", line 11, in <module>
        from socket import AF_BRIDGE
    ImportError: cannot import name AF_BRIDGE