Fix ringbuilder tests

Some of tests in test/unit/cli/test_ringbuilder doesn't assert
the exit code and unfortunately some of these passed even if the
statement fails for the assertion actually.

This patch enables to assert the exit code from ringbuider and
fixes some code/test bugs I noticed.

Change-Id: I18fa675ba8a90678e2b5ccb5f90eafab01d22787

swift/cli/ringbuilder.py

@@ -1183,7 +1183,7 @@ swift-ring-builder <builder_file> set_overload <overload>[%]
 
 def main(arguments=None):
     global argv, backup_dir, builder, builder_file, ring_file
-    if arguments:
+    if arguments is not None:
         argv = arguments
     else:
         argv = sys_argv