Fix magic in zuul_swift_upload

It turns out there are two implementations of libmagic for python.
PyPI has a wrapper (https://github.com/ahupp/python-magic) and
ubuntu packages the bindings from (https://github.com/file/file).

Update zuul_swift_upload to try the wrapper first and fall back to
the bindings. If both don't work an exception should be raised.

Change-Id: I248163d610b074c6de8a6408a6c458d3198d18cc