cleanup resources in setUpClass if exception raised

Resources created would not be cleanup if exception raised
in setUpClass.

This patch adds a new decorator to wrap setUpClass for
catching the exception in setUpClass and cleanup resources
using tearDownClass().

Use the decorator in the following test classes:
FloatingIPsTestJSON
ImagesMetadataTestJSON
ListServerFiltersTestJSON

Change-Id: Ia20ebd2a5d1b5ecbd5e8b21e51e7c2ee4c99fbd5
Closes-Bug: #1285509
Merge "cleanup resources in setUpClass if exception raised"