Replace unicode() for six.text_type

To support Python3, unicode() calls has been replaced by
six.text_type.
Added utils.exception_to_str(): is the proper way to convert
an exception to string, since it manages logic related to
encoding.

Change-Id: I27101390e4f95e5c7690b1b445b7e75b8bcb9a08
Closes-Bug: #1284677

glance/registry/api/v1/images.py

@@ -18,6 +18,7 @@ Reference implementation registry server WSGI controller
 """
 
 from oslo.config import cfg
+import six
 from webob import exc
 
 from glance.common import exception
@@ -481,7 +482,7 @@ class Controller(object):
                                    request=req,
                                    content_type='text/plain')
         except exception.Conflict as e:
-            LOG.info(unicode(e))
+            LOG.info(six.text_type(e))
             raise exc.HTTPConflict(body='Image operation conflicts',
                                    request=req,
                                    content_type='text/plain')