Fixed multiple py34 gate issues

Fixed multiple py34 gate issues

1. Scope mock of 'open' to module

By mocking 'open' at the module level, we can avoid affecting
'open' calls from other modules.

2. Stop using LOG.exception in contexts with no sys.exc_info set

Python 3.4 logger fills in record.exc_info with sys.exc_info() result
[1], and then it uses it to determine the current exception [2] to
append to the log message. Since there is no exception, exc_info[1] is
None, and we get AttributeError inside traceback module.

It's actually a bug in Python interpreter that it attempt to access the
attribute when there is no exception. It turns out that it's fixed in
latest master of cPython [3] (the intent of the patch does not seem
relevant, but it removes the offending code while reshuffling the code).
Note that now cPython correctly checks the exception value before
accessing its attributes [4].

The patch in cPython that resulted in the failure is [5] and is present
since initial Python 3k releases.

The patch in fixtures that broke us is [6].

[1]: https://hg.python.org/cpython/file/tip/Lib/logging/__init__.py#l1412
[2]: https://hg.python.org/cpython/file/tip/Lib/logging/__init__.py#l575
[3]: https://hg.python.org/cpython/rev/73afda5a4e4c
[4]: https://hg.python.org/cpython/rev/73afda5a4e4c#l6.484
[5]: https://hg.python.org/cpython/rev/2ee09afee126
[6]: https://github.com/testing-cabal/fixtures/commit/67dd2956943261e845a866dab155208c51da937e

Closes-Bug: #1503847
Closes-Bug: #1504053
Co-Authored-By: Ihar Hrachyshka <ihrachys@redhat.com>
Change-Id: I456b7846b8a53e4d3f8c91583685e0e1eaa84719