Allow clean_up with missing image ref

This change allows clean_up to continue with missing image_source
in Node.instance_info. This previouly failed in cases where:

* A node failed to deploy and so didn't have image_source
* A node was migrated from nova-baremetal

Change-Id: I03543adc74f9da83fff58e0cffe34f36a055e19a
Closes-Bug: 1368984
Merge "Allow clean_up with missing image ref"