Implement exception module i18n support

The doc string in exception.py of Keystone will be
returned with __doc__ method, but cannot realize the
internationalization.Change exception module to enable
i18n support.

Changes in the patch are:
1, useing class variable msg_fmt to replace class __doc__
2, modify wsgi.render_exception function using unicode
function to replace str function
3, modify/add UT test cases

Fixes: bug # 1179425

Change-Id: I75c1229c905a2625d2f6961d1a8dd3958eac51a5
Merge "Implement exception module i18n support"