use six.StringIO for compatibility with io.StringIO in python3
use six.StringIO for compatibility with io.StringIO in python3

The newer version of six (1.4.1) provides six.StringIO
which is a fake file object for textual data. It's an
alias for cStringIO.StringIO, in python2 and io.StringIO
in Python3

Change-Id: I5f4af05eb0754d85a73068d5f6b285d4be07cdfc

openstack/manila,openstack/manila,bswartz/manila,bswartz/manila,vponomaryov/manila,vponomaryov/manila