The format_exc method does not take an exception

For py35, this call blows up.  Seems to be ignored for py27, but
even in py27, it doesn't take an exception.

https://docs.python.org/2.7/library/traceback.html
https://docs.python.org/3/library/traceback.html

Change-Id: I2602426b966045b15b96e5e41d0df6524ed05119

openstackclient/shell.py

@@ -116,7 +116,7 @@ class OpenStackShell(app.App):
             if not logging.getLogger('').handlers:
                 logging.basicConfig()
             if self.dump_stack_trace:
-                self.log.error(traceback.format_exc(e))
+                self.log.error(traceback.format_exc())
             else:
                 self.log.error('Exception raised: ' + str(e))