Unit tests fail with cryptography < 1.6

Trying to run the unit tests with any version of cryptography < 1.6 fails with the following errors:

======================================================================
FAIL: magnumclient.tests.v1.test_bays_shell.ShellTest.test_bay_config_swarm_success_with_tls_non_csh
tags: worker-3
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Empty attachments:
  stderr
  stdout

Traceback (most recent call last):
  File "magnumclient/tests/v1/test_bays_shell.py", line 388, in test_bay_config_swarm_success_with_tls_non_csh
    tls_disable=False)
  File "/tmp/venv/lib/python2.7/site-packages/mock/mock.py", line 1305, in patched
    return func(*args, **keywargs)
  File "magnumclient/tests/v1/test_bays_shell.py", line 372, in _test_bay_config_success
    self._test_arg_success('bay-config test_bay')
  File "magnumclient/tests/v1/shell_test_base.py", line 91, in _test_arg_success
    stdout, stderr = self.shell(command)
  File "magnumclient/tests/utils.py", line 161, in shell
    _shell.main(argstr.split())
  File "magnumclient/shell.py", line 665, in main
    args.func(self.cs, args)
  File "<decorator-gen-22>", line 2, in do_bay_config
  File "magnumclient/common/cliutils.py", line 97, in wrapper
    return func(*args, **kwargs)
  File "magnumclient/v1/bays_shell.py", line 245, in do_bay_config
    tls = _generate_csr_and_key()
  File "magnumclient/v1/bays_shell.py", line 351, in _generate_csr_and_key
    backend=default_backend())
  File "/tmp/venv/lib/python2.7/site-packages/cryptography/hazmat/backends/__init__.py", line 41, in default_backend
    _default_backend = MultiBackend(_available_backends())
  File "/tmp/venv/lib/python2.7/site-packages/cryptography/hazmat/backends/multibackend.py", line 33, in __init__
    "Multibackend cannot be initialized with no backends. If you "
ValueError: Multibackend cannot be initialized with no backends. If you are seeing this error when trying to use default_backend() please try uninstalling and reinstalling cryptography.
Ran 351 tests in 1.108s (+0.055s)
FAILED (id=17, failures=8)

Based on https://github.com/openstack/python-magnumclient/blob/master/requirements.txt#L17, lower versions should be supported.