New base class for all OpenStack mock tests.

https://issues.apache.org/jira/browse/JCLOUDS-340

This is going to look a little odd because it doesn't appear to be used anywhere. It's actually being used by openstack-swift in jclouds-labs-openstack. But I can't submit my PR to jclouds-labs-openstack until this has been merged, otherwise the jclouds-labs-openstack tests will fail. Just a natural consequence of having different repos. Once the openstack-swift refactoring is done, I'll be able to use it for openstack-marconi too.