Fix warning log message typo in resource_tracker.instance_claim

Just cleans up a small typo in a warning log message that the resource
tracker gives when 'node' is already set on the instance during a claim
operation.

Change-Id: Ic1ece4451f54f4efc8028f8d90de5d240910b20e
Merge "Fix warning log message typo in resource_tracker.instance_claim"