Broken test in test_maas_certificates

[Dougal Matthews]

This test is currently failing and will be marked as a expected failure in https://code.launchpad.net/~d0ugal/maas/+git/maas/+merge/385351

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ERROR: provisioningserver.tests.test_maas_certificates.TestMAASCertificates.test_generate_certificate_if_needed
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testtools.testresult.real._StringException: Empty attachments:
  Twisted logs

Traceback (most recent call last):
  File "/home/ubuntu/code/maas/src/maastesting/runtest.py", line 126, in _run_user
    result = function(*args, **kwargs)
  File "/home/ubuntu/code/maas/.ve/lib/python3.6/site-packages/testtools/testcase.py", line 719, in _run_test_method
    return self._get_test_method()()
  File "/home/ubuntu/code/maas/src/provisioningserver/tests/test_maas_certificates.py", line 69, in test_generate_certificate_if_needed
    self.assertTrue(maas_certificates.generate_certificate_if_needed())
  File "/home/ubuntu/code/maas/src/provisioningserver/maas_certificates.py", line 117, in generate_certificate_if_needed
    elif _cert_mtime and _cert_mtime != os.path.getmtime(MAAS_CERTIFICATE):
  File "/usr/lib/python3.6/genericpath.py", line 55, in getmtime
    return os.stat(filename).st_mtime
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/tmpm5mo4klc/maas.crt'

[Alberto Donato (ack)]

This might be related to the globals in maas_certificates.py

_cert_not_before = None
_cert_not_after = None
_cert_mtime = None

We should investigate not caching those values, unless it's really expensive not to do so.