Ensure request_spec can be serialized.

Ensure request_spec can be serialized.

request_spec includes some objects that must be converted to primitive
types to ensure they can be serialized by any of the rpc drivers.  In
this case, it's the volume type.  It's a sqlalchemy model.
to_primitive() will ensure that it gets converted to a dict so that it
can be serialized for messaging.

I saw this error in a smokestack log that was running tempest with qpid.
You won't see this error if you're using rabbit with the kombu driver.
We have some magic with the kombu driver that will fix these problems
automatically if they are missed.

Change-Id: I9a1b7662bca7caa2acb61d1e3879ac2654d5b32e