Serialize instance object while building request_spec

The scheduler utility method for building a request_spec could be passed
an instance dict or instance object.  The jsonutils.to_primitive call on
the request_spec dict serializes an instance object but should not be
relied upon because it uses an interface on the object that may go away.
Instead, an object should be serialized properly before going into the
request_spec.

Change-Id: I9d810a0b8ee6f1beb79d831053f4bbd12b761b0b
Merge "Serialize instance object while building request_spec"