Make compute_api.reboot() use objects

This makes the compute api's reboot() method use objects and pass
them to compute/manager.

The one place we call back from compute/manager to compute/api's
reboot() is also converted to use objects in this patch. That
almost makes bug 1196301 go away, but will require instance
actions work as well.

This also adds a new cells method for passing the object to reboot in
the child cell.

Related to blueprint compute-api-objects

Change-Id: I7845daa266687b5dc24fc2b1a98f2e5a2841e4bf
Merge "Make compute_api.reboot() use objects"