Switch all uses of StringIO to use it from six

As part of enabling python3 support in tempest we need to be able to
handle StringIO usage in both python2 and python3. Six provides a
compat layer for doing this, so this commit moves all uses of StringIO
and cStringIO to get it through six.

Change-Id: Ie6ac86b3b5ed2c307dc2cc41386da9e5ba1ee23e
11 files changed