Unify StringIO usage with six.StringIO

Python 3 doesn't include StringIO.StringIO, but includes io.StringIO.
To unify StringIO usage, use six.StringIO instead.

See http://pythonhosted.org/six/#six.StringIO

Part of blueprint keystone-py3kcompat

Change-Id: Ieb03821ef6be2195ea2d58445de507dadee7e3d2
Merge "Unify StringIO usage with six.StringIO"