Updates the RPC cleanup function for the zmq RPC driver to use Context.term() instead of Context.destroy(). This allows the Nova code to work with older versions (pre 2.1.10) of pyzmq. The Context term() function seems to do the job just as well as destroy() (and is also thread safe) so lets just use it instead. Fixes LP Bug #1044004. Change-Id: I07a200d4b216c0c1e155b57588295b35a237576c