Remove use of str() in utils.misc.Failure

Recent updates to gettextutils in OpenStack add the
ability to translate errors in REST API responses.  The
new gettextutils code, however, requires that the error
strings passed to it be unicode, not basestrings.  A
basetring being sent from str() results in an exception
being raised.

This change switches from using str() to six.text_type()
in utils.misc.Failure to avoid problems after moving
Cinder to use the new lazy translation support.

Change-Id: I9a1e0ba714e9f3d214269c0f69726b0a1d1a4c7c
closes-bug: 1275895