Cleanup object usage in the rebuild path

This patch removes a redundant fetch of instance system_metadata
that was used to refresh our notion since the first get. Now that
the rest of the code is using instance.save(), it has an updated
view of system_metadata at effectively the same point.

Related to blueprint icehouse-objects

Change-Id: I357daa5ae627e8959b77bd0cb00e152810f9c0eb
Merge "Cleanup object usage in the rebuild path"