swift-ring-builder blows up when not supported command requested.

swift-ring-builder uses outdated unbounded class method to handle
not supported command. It worked for python 2.6 or older but not
for python 2.7 or newer. This patch fixes the problem.

Change-Id: I7dbc681ef6be44f6d79ff93189ccca13c51eab74
Fixes: bug #1154882
Merge "swift-ring-builder blows up when not supported command requested."