Make _poll_unconfirmed_resizes() use objects

This makes the _poll_unconfirmed_resizes() periodic task use
objects for looking up the instance state information.

This patch also makes compute_confirm_resize() take instance
objects. Although this method will be obsolete after a full
conversion to objects, this makes it send a full instance object
to the conductor manager, which is then converted to a dict for
passing to compute_api (for now). This will allow a more complete
conversion of the use of it to objects, and then the dict hack
will be removed when the method itself goes away.

Related to blueprint unified-object-model

Change-Id: I03c1a28a1bbd9732f803e140e93d214dd5c7792e
Merge "Make _poll_unconfirmed_resizes() use objects"