Replace exception_to_str with oslo.utils function

The oslo.utils function has exception_to_unicode that can
replace glance util function exception_to_str.
So we don't to have this exception_to_str function in glance
anymore.

Change-Id: I332bc55558087920fdd6ae2d822bece5166f5ba6

glanceclient/shell.py

@@ -765,4 +765,4 @@ def main():
     except KeyboardInterrupt:
         utils.exit('... terminating glance client', exit_code=130)
     except Exception as e:
-        utils.exit(utils.exception_to_str(e))
+        utils.exit(encodeutils.exception_to_unicode(e))