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
Add wsgiref to requirements.txt

Baremetal deploy helper (nova/cmd/baremetal_deploy_helper.py) uses
wsgiref module, so it should be in requirements.

Closes-bug: #1263480
Change-Id: Ibdd917d8065c085411a21a65e2d9ae016e265ee5
Merge "Add wsgiref to requirements.txt"
Merge "Cleanup object usage in the delete path"