The nova/tests/functional/api_sample_tests/test_servers.py contains the
ServersSampleBase class, and in its class definition creates a
'user_data' attribute by base64 encoding a string. However, this will
not work in Python 3, as the base64.b64encode() method requires bytes,
not a string. As a result, importing the test class fails and no tests
get run.
Note that this change doesn't fix all tests to work under Python 3; it
simply fixes the bug that prevents the tests from running at all under
Python 3.
Closes-Bug: #1632856
Change-Id: I35a7b02132bed0387a173b339f6204bf0e3269de
a2d7ebd39b