Handle instance objects in conductor compute_stop

The compute manager is sending instance objects to
compute_stop in conductor but conductor's compute_stop
RPC proxy wasn't handling objects. This patch fixes that
and also handles compatibility with older installations
using the dict-style format in the conductor compute_stop
RPC proxy.

Fixes bug 1195849

Change-Id: Ib678df424a58a0d7023ce3b2bdd5001998daf398
Merge "Handle instance objects in conductor compute_stop"