VimExceptions need to support i18n objects

[Davanum Srinivas (DIMS)]

When lazy is enabled the i18n translation object does not support
str() which causes failures like:
  UnicodeError: Message objects do not support str() because they may
  contain non-ascii characters. Please use unicode() or translate()
  instead.

[James Carey (jecarey)]

Here is the failure when i18n lazy translation is enabled in Nova (note not enabled in Juno):

nova.tests.virt.vmwareapi.test_driver_api.VMwareSudsTest.test_exception_with_deepcopy
-------------------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):
      File "nova/tests/virt/vmwareapi/test_driver_api.py", line 113, in test_exception_with_deepcopy
        copy.deepcopy, self.vim)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/testtools/testcase.py", line 420, in assertRaises
        self.assertThat(our_callable, matcher)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/testtools/testcase.py", line 431, in assertThat
        mismatch_error = self._matchHelper(matchee, matcher, message, verbose)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/testtools/testcase.py", line 481, in _matchHelper
        mismatch = matcher.match(matchee)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/testtools/matchers/_exception.py", line 108, in match
        mismatch = self.exception_matcher.match(exc_info)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/testtools/matchers/_higherorder.py", line 62, in match
        mismatch = matcher.match(matchee)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/testtools/testcase.py", line 412, in match
        reraise(*matchee)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/testtools/matchers/_exception.py", line 101, in match
        result = matchee()
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/testtools/testcase.py", line 965, in __call__
        return self._callable_object(*self._args, **self._kwargs)
      File "/usr/lib/python2.7/copy.py", line 174, in deepcopy
        y = copier(memo)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/oslo/vmware/service.py", line 201, in request_handler
        _("No such SOAP method %s.") % attr_name, excep)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/oslo/vmware/exceptions.py", line 52, in __init__
        self.msg = str(message)
      File "/opt/stack/nova/.tox/py27/local/lib/python2.7/site-packages/oslo/i18n/_message.py", line 167, in __str__
        raise UnicodeError(msg)
    UnicodeError: Message objects do not support str() because they may contain non-ascii characters. Please use unicode() or translate() instead.