Make rebuild use Instance objects

This makes the rebuild operation use Instance objects through to
the compute manager. It was already taking them from the rebuild
API code, but converting it to a dict almost immediately, before
it got passed to anything else. This cleans that up and makes it
native.

Related to blueprint compute-manager-objects

Change-Id: I46430af3a52f4fe2f8c2e3c2d654cd8eb0fa1662
Merge "Make rebuild use Instance objects"