Cleanup object usage in the delete path

This patch cleans up some dict-like usage of instances in the compute
api path, as well as removes a redundant direct call to fetch the
instance's system_metadata.

Related to blueprint icehouse-objects

Change-Id: I1cf89f0a46280b53d7d929ca61dde1cb3ea3cace
Merge "Cleanup object usage in the delete path"