Running 'limits show' returns nothing

Running limits show without --absolute or --rate returns nothing and
the user is left to figure out what they need to provide to get the
correct data back.  This patch prints an error and help output by making
at least one of the arguments required.

Change-Id: I576cf8ec0e05524ee67d46c48b56da8d44258667

Author: major

openstackclient/common/limits.py

@@ -31,7 +31,7 @@ class ShowLimits(lister.Lister):
 
     def get_parser(self, prog_name):
         parser = super(ShowLimits, self).get_parser(prog_name)
-        type_group = parser.add_mutually_exclusive_group()
+        type_group = parser.add_mutually_exclusive_group(required=True)
         type_group.add_argument(
             "--absolute",
             dest="is_absolute",