MAAS unit tests (at a minimum) are broken when bonding driver is loaded

[Mike Pontillo]

When running unit tests, the following error was seen:

http://paste.ubuntu.com/11727748/ -

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ERROR: metadataserver.deployment.tests.test_maas_configure_interfaces.TestMapInterfacesByMAC.test__integrates_with_real_sys_class_net
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_StringException: Traceback (most recent call last):
  File "/opt/src/maas/fix-bonding-test/src/metadataserver/deployment/tests/test_maas_configure_interfaces.py", line 181, in test__integrates_with_real_sys_class_net
    real_interfaces = script.map_interfaces_by_mac()
  File "/opt/src/maas/fix-bonding-test/src/metadataserver/deployment/maas_configure_interfaces.py", line 121, in map_interfaces_by_mac
    os.path.join('/sys/class/net', interface, 'address'))
  File "/opt/src/maas/fix-bonding-test/src/metadataserver/deployment/maas_configure_interfaces.py", line 100, in read_file
    with open(path, 'rb') as infile:
IOError: [Errno 20] Not a directory: u'/sys/class/net/bonding_masters/address'

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This could possibly occur in production as well (when the bonding driver is loaded), so we should fix this ASAP.